170 



ONYCHOPHORA. 



In the better preserved eggs figured by Sedgwick, large cavities can be seen 

 in the protoplasm, and this leads us to conjecture that, in the normal condition, 

 the eggs might be filled with a more or less fluid mass of yolk. These spaces in 

 the body of the egg are very large, occupying a large part of its interior, so that, 

 when the very unsatisfactory condition of the material investigated is taken into 

 account, we are led to the conclusion that the cleavage may in this case also be 

 superficial. The cavities in the blastula-stage just described would then be 

 filled with yolk, and the gastrula would perhaps be formed by invagination, 

 as was conjectured in the case of P. novac-zealandiac. As we have not personally 

 examined these eggs, wc do not feel justified in giving definite expression to this 

 view, but we cannot refrain from making a conjecture which appears to us so- 

 probable. There would in this case be a certain similarity between the African, 

 and the New Zealand species, especially as it may with probability be assumed 

 that eggs poor in yolk are to be derived from eggs rich in yolk. This last view 

 is held by Sedgwick himself, and in a later treatise (No. 10, Pt. iii.) he calls- 

 the egg of P. capensis meroblastic, because of the central connection mentioned 

 above as existing between the blastomeres. 



The American species. 



On account of the small size of their 



t»W«»flBM 



c oo° 









Us. 



f ~. E 



E. 



ire..._L' 



' " r^ -_-> f M "+~- ■"'&•'■&*• 



w 



t.Uro. - ' 



3P o - 



Pig. 70.— Sections through embryos of P. Edwardsii 

 together with the uterine wall (after J. v. Kennel). 

 E, embryo; am, amnion; a.Uw, outer wall of the 

 uterus; i.Uvj, inner wall of the uterus; Ue, uterine 

 epithelium [embryonic derivative, Sclater and 

 Willey]. 



and the connection 

 between these and the 

 wall of the uterus, the 

 American species differ 

 entirely in their develop- 

 ment from the forms we 

 have so far considered. 



The small eggs, poor in 

 yolk, undergo a total and 

 apparently fairly regular 

 (equal) course of cleavage. 

 The embryo, even at this 

 early stage, appears to 

 obtain nourishment from 

 the uterus, for it increases 

 in size in a marked manner 

 (v. Kennel). When it 

 has reached the 32-cell 

 stage, it forms, according 

 to v. Kennel, a solid cell- 

 mass, completely filling 

 the narrow lumen of the 

 uterus, and thus in close 

 contact with the inner 

 surface of the uterine 

 epithelium (Fig. 78). This 



