564 



ZOOLOGY 



SECT. 



In accordance with the smaller size of the ova and the rela- 

 tionship of the embryo with the wall of the uterus the American 

 species show a totally different mode of development. The eggs, 

 which are almost entirely devoid of yolk, undergo a total and 

 tolerably equal process of segmentation. Even at this stage the 

 embryo, which increases considerably in size, appears to receive 

 nutrient lymph from the uterine wall. When it has reached the 

 32-cell stage the embryo, according to one observer, consists of a 

 solid mass closely invested by the epithelium of the wall of the 

 uterus. It then becomes reduced in size and owing to exosmosis, 

 assumes the form of a disk placed in close apposition to one side 

 of the wall of the uterus. The embryo subsequently loses its 



FIG. 44(5. Two early stages in the development of Peripatus novce zealandiee. A, transverse 

 section of an ovum in which the yolk is nearly covered by the blastoderm (W.) ; B, transverse 

 section of an ovum in which the blastopore (blp.) is formed. (After Sheldon.) 



flattened form and becomes somewhat vesicular, the cavity of the 

 vesicle opening into the cavity of the uterus. From its surface 

 are given off isolated cells which become applied in part to th< 

 wall of the uterus, and finally unite to form a complete envelop 

 (amniori) enclosing the embryo. The vesicle then becomes closed, 

 the embryo becomes raised from the surface of the uterine wall, 

 the part applied to the latter becoming narrowed so as to form 

 sort of stalk, at the base of which is a growth of cells term* 

 the placenta. Into close relation with this placenta comes 

 ring-shaped thickening of the uterine wall, the uterine placenta. 



In P. capensis behind the elongated blastopore proliferation of 

 cells gives rise to an oval thickening. The mesoderm takes its 

 origin at this point and extends forwards in the form of two 

 germinal lands, one on the right of the blastopore and the other 

 on the left. These bands undergo a division into rudiments of 



