DEVELOPMENT OF THE EMBRYO. 15 



too black, and prevents them from ever being trans- 

 parent. 



The stages of the iuvagination of the blastula and 

 of the closing of the mouth of the gastrula can, I 

 am convinced, best be studied in the living object. 

 The reason is that the latter is sufficiently trans- 

 parent, and so in it the individual cells and cell nuclei 

 can be quite plainly distinguished. 



The use of reagents is again of importance for all 

 stages subsequent to, and inclusive of, that in which 

 the formation of the mesoblastic somites begins. In. 

 these stages the histological elements, as well as the 

 other details, cannot be distinguished with sufficient 

 clearness in the living object. With the picric sul- 

 phuric acid, which proved so useful for the segmenta- 

 tion stages, I could here obtain no satisfactory results. 

 On the other hand the use of osmic acid was here 

 fully justified. 



The treatment of the embryos was various, depend- 

 ing upon the end in view ; namely, whether the entire 

 object was to be studied, or the procuring of sections 

 was intended. 



I proceed now to discuss the treatment requisite 

 for the former of these purposes. The stages from the 



