XIII 



PHYLUM CHORDATA 



555 



we have seen, the seg- 

 mentation is of the in- 

 complete, or meroblastic 

 type, being confined to 

 a small disc of proto- 

 plasm the germinal 

 disc on one side of 

 the ovum. In the 

 Mammal, on the other 

 hand, except in the 

 Monotremes and some 

 Marsupials, segmenta- 

 tion is complete or 

 holoblastic, the entire 

 ovum taking part in 

 the process of seg- 

 mentation. The seg- 

 mentation is very ir- 

 regular, the cells into 

 which the ovum di- 

 vides being of unequal 

 size. The result is the 

 formation of a sphere 

 of cells, which soon 

 become distinguish- 

 able into an outer layer 

 and a central mass, the 

 embryonic cell-mass. By 

 imbibition of liquid a 

 cavity, which is formed 

 in the interior of the 

 ovum, increases rapidly 

 in size. The stage 

 now reached is called 

 the blastodermic vesicle. 

 During the growth in 

 size of the internal 

 cavity the central mass 

 of cells remains in con- 

 tact with one side only 

 of the outer layer, 

 where it spreads out j 

 as a stratum several 

 cells deep. It is in this 

 region the embryonic 

 area that the first 

 rudiments of the em- 

 bryo become developed. 



B 



eel 



mb.encL 

 -perLencl 



:o. 1146. Diagram representing sections of the embryo of a 

 Mammal at successive stages in the segmentation ana 

 formation of the layers. A and B, formation of enclosing 

 layer (trophoblast) and central cell-mass destined to give rise 

 to the embryo ; C, blastodermic vesicle with embryonic cell- 

 mass separated from trophoblast except on one side ; D, 

 blastodermic vesicle in which peripheral and embryonic por- 

 tions of endoderm have become established ; E, stage in which 

 the emWyonic ectoderm has broken through the trophoblast 

 and become joined to it peripherally, and in which the embry- 

 onic and peripheral endoderm have also become continuous. 



