DEVELOPMENT OF MAMMALIAN EMBRYO. 209 



these is, on the whole, very similar to that of Amphibia. 

 Until recently the growth of the mammalian embryo was 

 entirely wrongly explained ; and it is only lately (1875) 

 that Yan Beneden, whose views we adopt here, pointed out 

 its real sio^nificance.^^ His studies were directed towards 

 the embryo of the Rabbit, an animal in connection with 

 which Bischoff first discovered the history of the mamma- 

 lian germ. As the Rabbit in common with Man belongs to 

 the group of disco-placental Mammals, as this Rodent 

 develops entirely in the same way as does Man, and as even 

 at a later stage of evolution the embryos of Man and of the 

 Rabbit are hardly distinguishable (cf Plate VII. Fig. 

 K, 31), there is not the slightest reason to doubt that the 

 egg-cleavage and gastrulation of the two are similar. 



When the fertilization of the egg of the Rabbit is com- 

 plete, and the elaboration of the parent-kernel has trans- 

 formed the Monerula (Fig. 36) into the parent-cell, or cytula 

 (Fig. 37), the latter (the cytula) separates into the two first 

 cleavage-cells (Fig. 38). In this process the parent-kernel 

 first becomes fusiform and divides into two kernels (the 

 two first cleavage-kernels). These repel each other and the 

 two move apart. After this the protoplasm of the parent- 

 cell, attracted by the two kernels, parts into two halves, 

 each of which assumes a globular form. They afterwards 

 change from this globular to an ellipsoid form (Fig. 38). 

 These two cleavage-cells are not, as was formerly believed, 

 of the same size and significance. The one is larger, 

 brighter, and more transparent than the other. Again, the 

 smaller cleavage-cell takes a much deeper colour from car- 

 mine, osmium, etc., than does the larger. The two cells 

 thus already betray their relations to the two primitive 



