APP.] SEGMENTATION. 205 



Chromic acid is an excellent reagent for this and it 

 will be found very easy to make good sections. 



In the sections especial attention should be paid, 



1. To the first appearance of nuclei in the segments, 

 and their character. 



2. To the appearance of the horizontal furrows. 



3. As to whether new segments continue to be formed 

 outside the limits of the germinal disc, or whether 

 the fresh segmentation merely concerns the already 

 formed segments. 



4. In the later stages, to the smaller central and larger 

 peripheral segments, both containing nuclei. 



For surface views, the germinal disc, either fresh 

 or after it has been hardened, can be used. In 

 both cases it should be examined by a strong 

 reflected Hght. The chief point to be noticed is the 

 more rapid segmentation of the central than of the 

 peripheral spheres. 



IX. Examination of the later changes of the Embryo. 



For the later stages, and especially for the develop- 

 ment of the skull and the vascular system of the body 

 of the chick, it will be found necessary to dissect the 

 embryo. This can be done either with the fresh 

 embryo or more advantageously with embryos which 

 have been preserved in spirit. 



If the embryos are placed while still living into 

 spirit a natural injection may be obtained. And such 

 an injection is the best for following out the arrange- 

 ment of the blood-vessels. 



Sections of course will be available for study, 

 especially when combined with dissections. 



X. Study of the development of the Blood-vessels. 



Observations on this subject must be made with 

 blastoderms of between 30 — 40 hours. These are to 

 be removed from the egg, in the usual way (vide II. A. 

 and C.) spread out over a glass slip and examined 

 from below, vide II. E. 



