DEVELOPMENT OF THE EMBRYO. 17 



Moreover, staining, by making the objects less trans- 







parent than they should be, was seen to be only a 

 hindrance. For the later stages, however, I found 

 very useful a treatment with Beal's carmin, or 

 picrocarmin, following after the osmic acid, the 

 reason being that the distinctness of the forms was 

 considerably increased by the carmin. staining. In- 

 deed, in the stages subsequent to, and including, those 

 with eight mesoblastic somites, it may be regarded as 

 absolutely necessary. The objects stained with car- 

 min were in like manner cleared in glycerine. I 

 procured at the same time preparations preserved in 

 Canada balsam. 



I must now describe the treatment of the embryos, 

 supposing that the procuring of sections is the end 

 in view. I procured a number of serial sections, 

 beginning with that stago in which the mouth of the 

 gastrula becomes considerably smaller and the dorsal 

 surface flattened . From the early spherical stages, 

 optical sections of the living object can be obtained 

 in all directions, while the procuring of artificial 

 transverse sections is seen to be superfluous, yielding 

 as it does no new results. With a view to the pro- 

 curing of the sections, I first of all applied perosmic 



c 



