XIV ELEMENTS OF VERTEBRATE EMBRYOLOGY 467 



grade of adaptation to a purely terrestrial existence occurring in the eggs 

 of the majority of Reptiles and Birds, characterized (i) by the enclosure 

 of the egg in elaborate protective envelopes, (2) by the development of 

 an amniotic water-jacket to protect the embryo from jars, and (3) by 

 the hypertrophy of the precociously developed allantois to serve in the 

 first place as a reservoir for the renal secretion and in the second as a 

 bi-eathing organ. The higher grade of adaptation is found in the typical 

 Mammal, where the egg instead of being laid is retained within the genital 

 duct of the mother and leads therein a parasitic existence, so that the 

 young individual instead of, on the one hand, having to fend for itself 

 as in the lower vertebrates with free-living larvae, or on the other hand 

 developing in the not absolutely safe shelter of the egg-shell, forms as 

 it were, for the time being, simply a portion of an adult individual with 

 completely developed powers of looking after itself. Correlated with this 

 change in the conditions of development the egg of the mammal itself has 

 greatly diminished in size, a supply of yolk being no longer necessary, 

 and the elaborate protective envelopes so conspicuous in the reptile or 

 bird have been reduced to the verge of disappearance. The allantois 

 instead of performing a merely respiratory function performs a nutritive 

 one as well, playing a principal part in the formation of a complicated 

 organ — ^the placenta — for the extraction of nourishment from the mother. 



We will illustrate the peculiarities of the mammalian type of develop- 

 ment by the case of the Rabbit. 



The egg is a spherical cell about -i mm. in diameter. It is fertilized 

 in the upper part of the oviduct and as it passes onwards undergoes the 

 process of segmentation. In correlation with the disappearance of the 

 yolk the segmentation has reverted to the holoblastic type. The egg 

 divides into two, four, eight, and so on, the blastomeres being approxi- 

 mately equal. Eventually a solid sphere of cells is formed, consisting 

 of an inner mass of rather more granular-looking cells enclosed in an 

 outer layer ectodermal in its nature but given the special name trophoblast 

 from the fact that its physiological activities are devoted to the absorption 

 of nourishment. The egg now begins to increase rapidly in size, and 

 takes the form of a vesicle distended by fluid and walled in by the layer 

 of trophoblast, with the inner mass of cells adhering to its inner surface 

 at one pole. The blastocyst, as the hollow vesicle is now called, becomes 

 more and more distended with fluid, the trophoblast cells become broader 

 and thinner while the inner mass of cells also becomes extended out into 

 a comparatively thin layer covering, in the neighbourhood of the apical 

 pole, the inner surface of the trophoblast over a considerable area. This 

 layer of " inner-mass " cells now becomes differentiated into two layers — 



