ANIMALIA VErtTEBRATA— VERTEBRATED ANIMALS. 



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account lose their distinctive characters. Another cause of variety in the ultimate 

 development, while there is a perfect similarity in the primitive elements, arises from 

 this circumstance, that the different portions of these bones remain isolated in certain 

 species, while in others the greater number of scattered pieces become grouped to- 

 gether, and coalesce into one bone. Those modifications are particulai'ly realized in 

 the bones of the Skull; as it was especially necessary that this solid covering shodd 

 be modified so as to sxiit the peculiarities of the brain and the other essential organs, 

 which it was destined to enclose and to protect. 



This instance is not solitai-y; the other pieces of the skeleton are similarly modi- 

 fied in their arrangement, to correspond with the adjustment of the more important 

 organs. In the Fishes, for example, we find that the bones of the breast follow the 

 respiratory organs into their appropriate situation, and are grouped along with them 

 in the vicinity of the head. Without disturbing the order of Nature, it was impossible 

 that the air, when dissolved in water, could retain that elasticity belonging to its 

 gaseous form; it is unable to rnr-h by its pressure within the body of the fish, and to 

 seek out, as it were, the blood in the respiratory organ, for the purpose of purifying 

 it. The gills are accordingly placed in ibc vicinity of the jaws ; and it is only a con- 

 sequence of these remarkable laws of analogy, that they should be accompanied in their 

 places by the bones fitted for sustaining them, and by the muscles which enable thera 

 to act. These analogies, so obvious in the structure of the bones, may also be traced 

 in the nervous system of the Vertebrated Animals. 



It has been already observed, that the elements composing the bodies of the animals 

 belonging to the First Great Division are analogous ; but the analogy exists most per- 

 fectly only up to a certain period of animal development, and this period is not the 

 same in all, but varies according to the degree of complication in their structures. A 

 young unborn Jlammiferous quadruped resembles a Human embryo, which is less grown 

 than itself; but at the same time it resembles the embryo of a Bird in a higher state 

 of advancement, that of a Reptile in a still more perfect condition, and that of a 

 Fish, when completely finished, and perhaps already born. Thus, the classes of Ver- 

 tebrated Animals are perfectly analogous only at ages differing for each class. Fishes 

 and Reptiles may only resemble Birds and IMammalia which are much younger than 

 themselves. These last-mentioned animals, on the contrary, continue to grow, and 

 to increase in complication at a period of time long after the Reptiles and Fishes have 

 ceased to develop themselves. We thus see the cause of the greater complication of 

 structure among the higher animals. They are all identical in their first formation, 

 but the Reptiles continue their growth for a longer time than the Fishes, the Birds 

 longer than both Reptiles and Fishes, and Man and other IMammalia longer than these 

 three classes of Birds, Reptiles, and Fishes. 



Since all the Vcrtebrata can thus be brought to one common t}'pe, and to a base 

 nearly identical for all classes, it follows that each higher animal undergoes revolu- 

 tions analogous to those, observable in the whole series of animals inferior to it. One 

 single Vertebrated animal of the highest order, will hence exhibit in those transitory 

 states of existence, which pass from its first origin to its final completion, all those 

 cViaracters which are permanent arrangements in the lower classes of Vertebrated 

 Animals. It also arises, that the first ages of the human embryo form analof,ous 

 tj'pes to the other animals ; it first resembles a Fish, then successively a Reptile, Bird, 

 and finally a Mammiferous animal. Hence it appears, that in studying the development 

 of the human embryo, we are reading a minute, and, in its leading characters, an 

 accurate description of the physiology belonging to the higher classes of animated 

 nature. It has been often admired that so small and delicate an organ as the retina 

 of the human eye can exactly represent a distant and intricate scene with a precision 

 which the most perfect hmnan artist would in vain attempt to imitate; and shall we 

 not equally admire that minute and exquisite disposition of things, by which the 

 most complicated characters belonging to the distant inhabitants of the sea, of the land, 

 and of the air, are thus brought together and transcribed in miniature, so that a com- 

 plete history of a large portion of the Animal Kingdom is traced within the shell of an 

 egg, or the membranes of a human embryo ! 



This remarkable disposition of Nature requires further elucidation. 



The skin of the Human embryo precisely resembles that of certain Reptiles, or even 

 of the Medusffi and Polypi, by its softness, its perfect nakedness, as well as its sim- 

 plicity. In the earliest age, the anterior and central opening of the abdomen corres- 

 ponds to a disposition observable in the Oyster and some other Jlollusca, which have 

 the mantle divided daring the whole period of their life. At this time, likewise, the 

 muscles are colourless, soft, gelatinous, and destitute of tendons, as we may see 

 among the lower animals, such as the Worms. The bones of the human foetus arc 

 nearly rounded, as we find in adult animals of the lower grades. The same bone, 

 which is designed to form afterwards a whole, perfect in itself, is at this early period 

 broken up into as many separate points of ossification as we can find divisions, per- 

 manently separate among the I\Iammalia and Birds, but especially among the adult 

 Reptiles and Fishes. This singular correspondence of the temporary state of the 

 human bones with the permanent state of the same bones in the other Vertebrata, is 

 particularly observable in the occipital and sphenoid bones, in the upper jaw and tem- 

 poral bones, in many bones of the face, and also in the sternum, or breast-bone. The 

 breast-bone, for example, consists almost always of nine pieces in the first a^ cs of the 

 human embryo, and nine pieces are to be found in the Tortoise during the whole 

 course of its life. The upper jaw-bone is at first composed of five pieces in the human 

 fcetus, and the Crocodile continually preserves these five pieces isolated. We might 

 bring forward many other parallel instances of analogy among ages and classes, by 

 entering more minutely into the details of the skeleton. It is only in consequence of this 

 law that the bones of inferior animals ai-e more numerous than those of animals more 

 elevated than themselves, and that the animal skeleton exhibits a greater number of 

 bony pieces, as we approach the first periods of ossification. Indeed, there is almost 

 as much difference between the skeletons of the human fietus and that of the fuU-growu 

 Man, as between those of the adult Reptile, and of a Mammiferous Quadruped m its 

 embryo state. 



We shall find that these analogies, between the permanent states of the inferior 

 classes, and the transitory states of embryos of the higher animals, can be traced equally 

 m the heart and circulating organs, in the lungs and rcspii-atory organs, in the nervous 

 S 



system, and in the organs of sense, as well as in the arrangements finally destined to 

 continue the several species. 



The heart of the human foetus is first composed of one single cavity. It after- 

 wards divides into four, which form momentarily a communication with each other, 

 but speedily separate, in such a manner that the two cavities of the left side end in 

 having no direct communication with the cavities on the right. This sketch of the 

 progress of the human heart, which is equally applicable to that of the other Mam- 

 malia and of Birds, indicates a new analogy between the very young human embryo, 

 still destitute of a heart, and the Worms, which never have a heart. Again, when 

 the heart of the human embryo has only one ventricle, it resembles that of the Arach- 

 nides and Crustacea ; afterwards, the heart with two cavities corresponds to those of 

 the Fishes and Batrachian Reptiles. Finally, when there are but three cavities, the 

 two auricles having coalesced into one, it resembles the hearts of the Tortoises and 

 Serpents ; and when the partition of the auricles is pierced by the hole termed the 

 foramen ovale, the heart of the human foetus bears a striking analogy to a permanent 

 ai'rangement found in the Seals. 



In following out this comparison of the progress of the human embryo, with the 

 permanent arrangement of the adult animals, we see the venous blood of the human 

 fcetus communicate primitively with the arterial blood, which is a natural arrangement 

 in all the inferior animals, beginning from the Birds. The digestive canal is at first 

 short and simple, as we find in animals of the lowest degree. The liver is originally 

 composed of httle compartments, as may be seen continually in the Crustacea; 

 afterwards, it resembles that of the Jloliusca, in being formed of lobes slightly united. 

 The spleen and thymus gland are always absent among the Invertebrated Animals, 

 and the latter is wanting even in the Fishes. These organs are developed very late 

 in the embryo of Man and other Mammalia. The same remark applies to the breast- 

 bone. It is wanting in many Reptiles and in all the Fishes, and it is very late in 

 making its appearance in the embryo of the lai'ger animals. In general, we find that 

 those organs, of which the lower animals are altogether destitute, are the slowest in 

 being developed in the human foetus. On the contrary, the greater number of the 

 organs which exist only temporai'ily in the human foetus ai-e the first to appear; and 

 thus the gills, which in our species appear in its very earliest state, soon vanish ; and 

 that kind of tail, which may be seen in the human embryo when forty days old, does 

 not exist longer than the fiftieth day. 



The kidneys of the foatus among the Mammalia are always very large, v/hich ar- 

 rangement is found to remain continually witli the Fishes. They are at first lobed 

 and of an unequal surface in the human embryo, nearly resembhug that form which 

 may be seen in the adult Fishes, Birds, and in many Reptiles and Mammalia. The 

 subrenal capsules are at first very large in the human embryo, a disposition which 

 exists in the Apes, and iri several adult Rodentia, such as the Squirrel and Mouse. 

 There appears also to exist a cloaca in the embryo of Man and of the Mammalia, as 

 may be invariably found during the whole Uves of the oviparous animals, and in the 

 Monotreinata of M. Geoffroy-Saint-Hihiire. 



We see corresponding analogies in the instruments of respiration, and in the man- 

 ner in which this function is performed. Th.3 Birds commence by breathing through 

 means of the membraneous filaments of the allantois, just as the PolTOi respire through 

 the skin. The foitus of iNlan, at first, has gills resembling those of the Fishes ; and 

 the Batra'.:hian Reptiles, before they have lungs fit for acting, breathe throu-'h gills 

 hke the Crustacea; Birds breathe through simple membranes before they respire 

 through lungs; and Man receives, through the placenta, blood already purified, as 

 long as his lungs cannot gain access to the air of the atmosphere. There are even 

 some animals, such as certain Reptiles during their metamorphoses, which successively 

 present all these different modes of respiration, and which thus resemble in tui-n all 

 the different classes of animals, excepting the Insects, breathing throuo-h trachuje. 

 These Reptiles respire, at first, through a naked skin hUc the Polypi; afterwards 

 through external gills, like the Crustacea and Annelides; after that again, throu'^h 

 internal gills, like the Fishes; and finally, through lungs, in the same manner as the 

 other animals of their own and superior classes. 



But these analogies among the transitory states of -the foetus and the permanent 

 forms of the lower animals, when in a state of perfection, are in no respect more evi- 

 dent than in the details of the nervous system. Thus the human foetus, which we 

 have selected as forming the standard of comparison, has at first certain parts of the 

 brain (corpora quadriyemina) exactly similar to those belonging to the perfect Rep- 

 tiles and Fishes. These organs were originally hollow, lobed, not quadruple, but 

 double only, and placed merely on the surface of the enkephalon ; yet they finally as- 

 sume the permanent form of the Reptiles and Fishes. The Mammalia are the only 

 animals in which these tubercles become quadruple, and they are the only ones where 

 they become solidified by the obliteration of the central cavity. Besides the above- 

 mentioned analogy in regard to the nervous system, the human embryo presents many 

 other pecuharities corresponding to those of the lower animals. For example, the 

 hemispheres of the brain are possessed at first of only a small volume, and they are 

 rounded in a manner resembling the adult Fishes. Certain parts of its brain (cor- 

 pora callosa) are so divided that they appear at first sight to be absent, in the same 

 manner as with the Birds of every age. 



The spinal marrow of the human embryo exhibits a central cavity; and a similar 

 arrangement is found in all perfect animals of classes inferior to the Mammalia, while 

 even the lateral nervous cords are originally so much isolated, as to give to the entire 

 structure the appearance whieh it always preserves in the Articulated animals. Be- 

 sides thia, the marrow of the human embryo originally occupies the entire lcn<Ttli of 

 the vertebral canal, as in the other animals, but it is only in the third month that it 

 ascends as high as the loins. Finally, its entire nervous system esliibits several ana- 

 logies to the permanent dispositions in the other classes of Vertebrata, such as analo- 

 gies of consistency, volume, greater or less numerous subdivisions, and even of facul- 

 ties. 



In regard to the organs of sense, analogies of a similar order may be traced. The 

 mouth of the human foetus is at first without lips, as in the Vertebrated Animals of 

 the inferior classes. Its palate is then divided, and the mouth, on that account, 

 communicates directly with the nasal foss;^, as in the Reptiles and Bii-ds. The 



