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 Van Beneden, whose views we adopt here, pointed out 

 its real significance.^^ 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 



