5^4 



SEGMENtATION IN RABBlt 



ChAP. 



cntiated into an outer ectoderm and an inner, lower layer of cells 

 (compare Fig. 119), between which and the yolk the enteric cavity is 

 formed : a segmentation-cavity is hardly recognisible. As the embryo 

 develops, it becomes folded off from the yolk, which forms a yolk-sac on 

 its ventral side (Figs. 120 and 154). 



The minute egg of the rabbit and of most other Mammals, although 

 alecilhal and undergoing a holoblastic segmentation, has presumably 



Fig. 146. — Oosperm of rabbit 70-go hours after impregnation. 

 l/v, cavity of blastodermic vesicle (yolk-sac) ; ep. outer layer of cells (Irophoblast) ; 

 hy. inner mass of cells of the embryonic area ; Zp. albuminous envelope. (From 

 Balfour, after E. van Beneden.) 



been derived from a meroblastic type with abundant yolk like that 

 of the bird, and some Mammals living in Australia at the present day 

 still possess eggs of this type. In the higher Mammalia the yolk has 

 disappeared, as it is no longer needed, the embryo, as we have seen, 

 being nourished by means of a placenta, which will be described pre- 

 sently. The early processes of development are therefore somewhat 

 peculiar, and though the segmentation is holoblastic, the subsequent 

 development is essentially similar to that of the bird, the embryo 

 beginning lo appear in a mass of cells (Fig 146, hy) attached to 



