DEVELOPMENT OP THE EMBRYO. 987 



classes of that sub-kingdom, that we can discover whether the germ in course of 

 evolution is to become a Mammal, Bird, Reptile, or Fish. So, even when it 

 has been recognized as belonging to the Mammalian class, there is, at first, no- 

 thing to distinguish it from that of any other Mammal ; and it is only with 

 the advance of the developmental process that indications successively present 

 themselves, which enable us to distinguish, one after another, the characters of 

 the order, the family, the genus, the species, the variety, the sex, and the indi- 

 vidual the more special features progressively evolving themselves out of the more 

 general, which is the expression of the law of development, common to all Or- 

 ganized beings. 1 



994. With this progressive alteration in the condition of the embryo itself, a 

 very remarkable series of alterations takes place, pari passu, in the mode in 

 which it is supplied with nutrient material, and in the provisions for the aera- 

 tion of its circulating fluid. The first evolution of the germ takes place en- 

 tirely at the expense of the yelk ; of which, however, the store contained in 

 the Mammalian ovum is very small. The whole of this is very speedily incor- 

 porated in the substance of the germ, by the peculiar process to be presently 

 described ; and there is no residual store of " food-yelk/' such as that which, 

 in the Bird, serves for the nutrition of the embryo during the whole remainder 

 of the developmental process, being gradually absorbed into the substance of 

 the blastodermic membrane, and there converted into blood. The Mammalian 

 ovum, however, from the time it reaches the Uterus, is furnished with a new 

 supply of nourishment in the fluid secreted by the Decidual membrane ( 977) ; 

 and for the absorption of this, it is particularly adapted by the villosities which 

 develop themselves from its own external envelop. These, at first entirely des- 

 titute of bloodvessels, are subsequently penetrated at a certain part of the sur- 

 face by the fostal capillaries brought to them by an organ, the Allantois, which 

 is developed in Birds as the temporary instrument of respiration ; and thus is 

 originated the foetal portion of the Placenta, of whose formation details will be 

 presently given. From the time that this organ is completed, up to the birth 

 of the Infant, the embryo draws its nutrient materials direct from the maternal 

 blood, though not receiving that blood as such into its own organism ; and it is 

 through the same medium that the aeration of its own blood is effected, its pul- 

 monary apparatus being as yet inoperative. Its circulating system, arranged in 

 accordance with these requirements, presents many peculiarities which mark its 

 fo3tal character ; and the alteration in the course of the blood, which takes place 

 as soon as the respiratory organs come into play, constitutes the essential differ- 

 ence between intra-uterine and extra-uterine life. If, as sometimes happens, 

 the lungs of the new-born infant expand but imperfectly or scarcely at all, the 

 circulation continues to be carried on, in a greater or less degree, upon its intra- 

 uterine plan ; and this, when the placenta is no longer capable of supplying 

 the needed aeration, is incompatible with the persistence of life. 



995. Our knowledge of the first stages of the developmental process in the 

 Mammalian ovum is in many respects incomplete ; and it is requisite to inter- 

 pret what has been obscurely seen in the ova of this class, by the clearer views 

 derived from observation of those of the lower animals. 3 As already stated, 



1 See, on this subject, the Author's "Princ. of Phys., Gen. and Comp.," CHAPS, vnr. 

 and xviii., Am. Ed. ; in the former of which will be found an examination of the commonly 

 received doctrine, that the higher forms in the course of their development pass through 

 the phases which remain permanently characteristic of the lower. 



2 The researches of Kolliker (" Muller's Archiv.," 1843, p. 68), and Bagge (" DeEvolut. 

 Strongyli et Ascarid., Diss. Inaug.," 1841) on the ova of Entozoa those of Mr. Newport 

 ("Philos. Transact.," 1851) on the ova of Batrachia, and those of Bischoff ("Entwicke- 

 lungsgeschichte des Hunde-eies," 1845) on the ova of the Bitch are the most valuable 

 which we at present possess. 



