DEVELOPMENT OF MAMMALIA. 



427 



which it remains thick. This thick mass of granular cells we 

 caU the embryonic or germinal area. During the following 

 few days changes take place in the inner layer. About five 

 days after impregnation the cells of the embryonic area divide 

 into two layers, the lower composed of flattened cells, and forms 

 the hypoblast. During the subsequent day a middle layer 



Fio. 202.— OvDM OF Babbit. 



1, Optical section of a rabbit's ovum at a stage closely following segmentation ; Bl, 

 blastopore ; Hy, inner layer ; Ep^ outer layer. 2, Rabbit's ovum between 70-90 hours 

 after impregnation : Bl.v, cavity of blastodermic vesicle (yolk-sac) ; Bp, outer layer ; 

 Hy, inner layer ; Alb.B, albuminous envelope. (After E van Beneden.) 



arises and fuses with the epiblast to produce the true epiblastic 

 layer. These two layers form a circular embryonic patch on 

 the ovum, which on the seventh day becomes oval. There ap- 

 pears at the lower part of this spot a primitive streak as in 

 the fowl. The area then becomes pyriform, and the primitive 

 streak is marked by a groove. The epiblastic cells which form 

 the streak also give rise to part of the mesoblast. 



Very similarly to what we saw in the development of the 

 chick the medullary groove arises in front of the primitive 

 streak. The medullary plate between the two medullary folds 

 is composed of one row of epiblastic cells, only, at first. The 

 mesoblast arises from a double origin — (1) from the epiblast 

 and (2) from the hypoblast. The notochord arises as a 



