Z AET. 11. — S. HATTA. 



ova during gastrulation ; and these steps are, so far as it seems 

 to me, very peculiar but none the less interesting. I will describe, 

 in the following j)ages, the chief points obtained. 



Before passing to the explanation of the processes, it should 

 specially be mentioned here that the ova in the earlier stages, from 

 the blastula to the gastrula inclusive, are very liable to suffer 

 artificial injuries caused by the effects of the killing reagents 

 employed. At any rate, some unequal shrinkage is, in the stages 

 in question, almost unavoidable : the animal hemisphere which is 

 not only formed of a thin layer, but contains the hollow seg- 

 mentation cavity, contracts necessarily more severely than the 

 solid vegetative hemisjohere, thereby causing not a little alteration 

 in the shape of the ova. It is, therefore, advisable that the living 

 material should be employed, at least, for the study of the 

 superficial changes ; and it follows that great care should also be 

 taken in examination of sections« for the preparation of which 

 hardened ova are of course necessary. I have endeavoured to show 

 such shrinkage in some of the figures given {Figs. 17 and 21), 

 in which the ovum itself had considerably contracted causing strong 

 wrinkles in the non-contractile chorion. It is probably from the 

 same cause that hardened ova often deviate in configuration from 

 the living. 



For the sake of simplicity, I will start with what I have 

 come to consider as the old morula stage given in Fig. 1. . The 

 ovum at this stage presents the shape of a sphere, the lower third 

 of which shows a solid opaque appearance and represents the so- 

 called vegetative or yolk hemisphere ; while the upper larger part, 

 the animal hemisphere, is more or less translucent containing 

 the hollow segmentation cavity within. By stages, the ovum not 

 only increases in bulk, but assumes the outline of a tall ellipse 



