ANIMALIA VEllTEP.RATA— VERTEBRATED ANIMALS. 
29 
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 dllToront portions of these honeys remain isolated in certain 
species, while in others the greater iiumher of scattered pieces become grouped to- 
gether, and coalesce into one hone. These modifications are pariicularly realized in 
the bones of the Skull; as it was especially necessary that this soliil covering should 
Iw modified so as to suit the pijculiaritics of the brain and the other essential organs, 
w hich it was destined to enclose and to protect. 
This instonco ts not solitary; the other pieces of the skeleton arc similarly modi- 
fied in their arrangement, to eoi respond 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 w'illi them 
in the vicinity of the head. AVithout disturbing the order of Nature, it was inipossiblo 
that tho air, when dissolved in water, could retain that cluslieity bedonging to its 
gaseous form; it la unablo to rush by its pressure within the body of the fish, and to 
seek out, as it were, tho blood in tlio ri'.spiratory organ, for the purpose of purifying 
it. The gills ai'c accordingly placed in the vicinity of the jaws and it is only a <ron- 
scquencoof these remarkable laws of analogy, that they should he accompanied in their 
places by the bones fitted for sustaining them, and by tbo muscles which enable them 
to act. These analogies, so obvious in tho structure of the bones, may also be traced 
in tho nervous system of the Vertebrated Animals. 
Tt has been .already observed, that the elements composing tho bodies of the animals 
belonging to tho First Great Division arc analogous ; but the analogy exists most per- 
fectly on'j' up to a ccvtahi period of animal development, and this period is not the 
same in nil, but varies according to tho degree of complication in their structures. A 
young imlurn i\Iammiferous quadruped resembles a Human embryo, which is less grov^'n 
than itself ; but at the same lime it resembles tho emhryo of a Bird in a higher state 
of advancement, that of a Reptile in a still more ])crfcct condition, and that of a 
Fish, when completely finished, and perhaps already born. Tims, the classes of Ver- 
tebrated Animals arc perfectly analogous only at ages ibfFering for each class. Fishes 
and Reptiles may only rosomble Birds and Mammalia which ore much younger than 
themselves. These last-mentioned animals, on tho contrary, continue to grow, and 
to increase in complication at a period of time long after llic Reptiles and Fishes have 
ceased to develop themselves. ^Ve thus sec the cause of the greater complication of 
structure among the higher animals. They arc all identical in their first formation, 
but the Reptiles continue their growth for' a longer time than the Fishes, the Birds 
longer than both Rqitilcs and Fishes, and ^fan and other Mammalia longer than these 
three classes of Birds, Rojitilcs, and Fishes. 
Since all the Vertebrata can thus bo brought to one common type, and to a base 
nearly identical for ad chisses, it follows that each higher animal undergoes revolu- 
tions analogous to those, observable in tho whole series of animals inferior to it. One 
tingle Vcrtebratctl animal of the highest order, will hence exhibit in those transitory 
states of existence, whicli pass from its first origin to its find completion, all those 
characters whicli are permanent arrangements in tho lower classes cf Vertebrated 
Animals. It also arises, Uiat the first ages of the human embryo form analogous 
types to the other unimals ; it first resembles a Fish, then successively a Reptile, Bird, 
and finally a Mauuiiiferous animal. 1 lence it appears, that in studying tho development 
of the human embryo, wc arc reading a minute, and, in its leading characters, an 
accurate description of tho physiology belonging to tho higher classes of animated 
nature. It has been ofton admired that so small and delicate an organ as tho retina 
of the human eye can exactly represent a distant and intricate scone with a precision 
which tho most perfect human artist would in vjuu attempt to imitate; and sliall wc 
not equally admire that minute and exquisite disposition of thingg, by which the 
most complicated characters belonging to tho distant inhabitants of the sea, of the hind, 
and of the air, are thus brought together and transcribed in miniature, so that a com- 
plete history of a largo portion of the Animal Kingdom is traced within tho shell of an 
egg, or the membranes of a human embryo ! 
This rem.irkable disposition of Naturo requires further elucidation. 
Tho skin of the Human embryo precisely resembles that of certain Reptiles, or even 
of the Mcdusffi and Polypi, by its softness, its perfect nakedness, as well as it.s tim- 
pUcity. In the earliest age, the anterior and central opening of the abdomen corres- 
ponds to a dispoMtion observable in the Oyster and some other Mollusca, which have 
the mantle divided during tho whole period of their life. At this time, likewise, the 
muscles are colourless, soft, gelatinous, and destitute of tendons, as wc may sec 
among the lower animals, such as ttio Worms, The hones of tlie human fajtus arc 
•nearly rounded, as wo find in adult animals of the lower gmdos. Tiic 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 oasificalion as wc can find divisions, per- 
tnanently separate among the Mammalia and Birds, but especially among the adult 
Reptiles and Fishes. This singulai' correspondenco of tho temporary state of the 
bunan bones with the permanent slate of the same bones in the other Vertebrata, is 
particularly observable in tho occipital anvl .sphenoid bones, in tho upper jaw and tem- 
poral bones, in many bones of the face, and also in the sternum, or brc.ast-bono. Tho 
broast-bone, for example, consists almost always of nine pieces in the first ages of llio 
buinan embryo, oml nine pieces are to bu found in the Tortoise during the whole 
course of its life. Tho upper jaw’-boiie is at fir.it composed of five pieces in tho huuian 
lujtus, and the Crocodile conlimmlly preserves tlicse five pieces isolated. might 
^’’ing forward many other parallel instances of analogy among ages and classes, by 
’'’Jtoring more minutely into the details of the skeleton. It is only in consequence of this 
that the bones of inferior animals are more numerous than those of animals more 
♦elevated than themselves, and that tho animal skeleton exhibits a greater number of 
bony pieces, as we approach the fir.si periods of o-isification. Indeed, there is almost 
^ much difforcnce between the skeletons of the human foetus and that of the full-grown 
Man, as between those of the adult Reptile, and of a JIammiferous Quadruped in its 
ombryo state. 
We shall find that these analogies, between the permanent states of the inferior 
•lasses, and the transitory states of embryos of the higher animals, can bo traced equally 
the heart and circulating organs, in the lungs and respiratory organs, in the nervous 
S 
system, and in the organs of sense, as well as in tho arrangements finally destined to 
continue the several species. 
The heart of the human foetus is first composed of one single cavity. It aftcr- 
w.ards 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 wutU the cavities on the riglit. This sketch of the 
progress of the human heart, which is equally applicable to that of the other l\Iam- 
malia and of Birds, indicates a new analogy between the very young human embryo, 
still destitute of :i heart, and the Worms, w liich never have a heart. Again, when 
the heart of the human embryo has only one ventricle, it resembles that of the Avach- 
nides atid Crustacea; afterwards, the lieart witli two cavities corrcBponds to those of 
tho Fishes and Batrachian Iloptiles. Finally, when there .iro but three cavities, the 
two auricles having coalesced into one, it resembles the hearts of ihc Tortoises and 
Scr]ients; and when tho ]'urtitiun of tlie auricles is pierced by the hole termed the 
foramen ovule, tliu heart of the human fojtus bears a striking analogy to a permanent 
arrangement found in the Seals. 
In following out this comparison of the progress of the human embryo, with the 
permanent Qirangement of the adult animals, we see the venous blood of llic human 
fojtus communicate prindtividy with the arterial blood, which is a natural arrangement 
in all the inferior animals, beginning from the Birds. T‘ha digestive canal is at first 
short and simple, as we find in animals of the lowest degree. The liver is originally 
composed of little compyrtments, as may be seen conLiuually in the Crustacea; 
afterwards, it resembles that of the Jloliusca, in being formed of iobes slightly united. 
Tho spleen and thymus gland arc alwav's absent among [he Invertebvated Animals, 
and the latter is wanting even in the Fish»*s. These organs are developed very late 
in the embryo of Man and other IMammalia. Tlie .sam5 remark iqqdies to tho breast- 
bone. It is wanting in many Ib-ptiles and in all the Fishes, and it is very late in 
making its appearance in the embryo of the larger animals. In general, we find that 
those organa, of which the lower animals are altogether destitute, arc the slowest in 
being developed in the human foitus. On the contrary, the greater number of tho 
organs which exist only temporarily in the human fmtus arc the first to appear; and 
tlms 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 oaibryo whoa forty days old, docs 
not exist longer than the fiftieth day. 
The kidneys of the ftetus among the Mammalia arc ahvays very large, which ar- 
rangement is found to rcmiun continually with the Fishes. Timy are. at first lobed 
and of an unequal surface in the human embryo, nearly resembling that form which 
may be seen in the adult Fishog, Birds, and in many Reptiles and Mammalia Tlie 
subronal cnpsuk*s ai'C at first very Iru-go in tho huuian embryo, a disposition whicli 
exists in the Apes, and in several adult Rodcutia, such as tho Squirrel and Mouse. 
There appears also to exist a chmea in the embryo of Man and of the Mammalia, as 
may bo invariably found during the vvholo lives of the oviparous animals, and in the 
Monotremata of il. Gcoffroy-Saint-IIilairc. 
AVe SCO corresponding analogies in the instruments of respiration, and in tho man- 
ner in which this function is perfonnod. Tlw Birds coninicnee by brcalhing through 
mcan.s of ihc membnuiuous filameuta of the allantois, just ns tho Polyjii respire through 
the skin. The foitus of hlan, at first, has gills resembling those of the Fishes' and 
the Batrachian Reptiles, before they have lungs fit for acting, breathe through gills 
like ihc Crustacea; Birds breathe through .simple membranes before they respire 
through lungs; and Miui receives, througli the placenta, blood .vh^eady purified, .as 
long as his lungs cannot gain access to the air of the atmosphere. T'here arc even 
some animal.i, such as curtain Reptiles during ihoir metamorphoses, which Htmcessively 
present all these diiTcrent modes of respiration, and wliich thus resemble in turn all 
the diirerent classes of animals, excepting the Insects, bi‘c.alhiiig tlirough trachea?. 
These Reptiles respire, at first, through a naked skin like the FalypI; aftcrward.'i, 
througli extcniiil gills, like the Crustacea and Auneluies; after that again, through 
internal gills, like the Fishes; and finally, througli lungs, in the same manner as the 
other animals of their own and superior classes. 
But these analogies among the transitory states of the fmtus and the permanent 
forms of the lower animals, when in a state of perfection, arc in no respect more evi- 
dent than in tho details of tho nervous system. Thus the human fuitus, which we 
have selected as forming the standard of comparison, has at first certain parts of the 
brain (corpora (j^uaf.hitjcmiJiu) exactly similar to tbose belonging to the perfect Rep- 
tiles and Fishes. These organs were originally hollow, lobed, not quadruple, but 
double only, and placed merely rm the surface of the ciikephalon ; yet they finally as- 
sume the permanent form of the Reptiles and Fishes. The Mammalia are the only 
animals In which those tubercles become quadruple, and they are the only ones where 
they become solidified by tho obliteration of the central cavity. Besides the above- 
mentioned analogy in regard to the nervous system, tho human embryo presents many 
other peculiarities corresponding to those of the lower animals. For example, the 
hemispheres of the brain are possessed 'at first of only a small vtfiume, and they arc 
rounded in a manner resembling the adult Fishes. Certain parts of its brain (cor^ 
jiora callosa) arc so divided that tliey appear at first sight to ho absent, in the same 
luannor as with tho Birds of every age. 
T'he spinal maiTow of the human embryo exhibits a central cavity; and a similar 
arvangemant is found in all perfect animals of classes inferior to tho Mammalia, while 
even the lateral nervous cords arc originally so much isolated, as to give to the entire 
structure the appoarauco which it always preserv'cs in the Articulated animals. Be- 
sides this, the marrow of the human embryo originally occupies tho entire length of 
tho vertebral canal, as in the oilier animals, but it is only in the third month that it 
ascends as high as tho loins. Finally, its entire nervous system exhibits several ana- 
logies to the permauent dispositions in the other classes of Vertebrata, such as analo- 
gies of consistency, volume, greater or leas 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 fa-tus is at first without lips, as in tho Vertebrated Animal-s of 
tho inferior classes. Its palate is then divided, and ihc mouth, on that accuuid, 
communicates directly with the nasal fossa?, as in the Reptiles and Birii. T fie 
