418 



Attention is drawn to two figures of sections through the tail of 

 a 9 mm embryo. One shows how, in the older embryos, the hypoblast 

 cells modify their shape in the presence of the hypochordal rudiment 

 (Fig. 9). The other shows the form of the hypochord in the tail and 

 the position that is taken up by the dorsal aorta (Fig. 10). 



The next embryo described is of length 10 mm. Throughout the 

 trunk and most of the tail the hypochord is entirely freed from the 

 hypoblast; in the branchial region and head it is attached only over 

 two short areas; and at the posterior end chorda-hypoblast is still 

 being proliferated and raised out of the hypoblast. Degeneration of 

 the hypochord is now certainly shown over a small area in the trunk. 



A section through a point immediately behind the posterior end 

 of the cut-olf notochord presents three groups of cells marked out 





Fig. 9 



Fig. 11. 



Fig. 9. 9 mm embryo. Through posterior end of cut-off notochord. X 400. 

 Fig. 10. 9 mm embryo. Through cloacal region. X 400. ao dorsal aorta. 

 Fig. 11. 10 mm embryo. Behind posterior end of cut-off notochord. X 450. 



by the size and depth of staining of their nuclei, while the two upper- 

 most groups are distinguished from the other by their more deeply- 

 stained cytoplasm (Fig. 11, in which an artefact is shown on one side). 

 The notochordal cells have enlarged since they were proliferated and 

 resemble cells active in secretion. The hypochordal cells imitate them 

 in general appearance; but they are smaller and apparently less ac- 

 tive. Below are hypoblast cells. Though the hypochordal cells are 

 here somewhat exceptionally developed, they illustrate a tendency for 

 those latest formed to travel the farthest along the some path as the 

 notochordal cells. The hypochordal cells the earliest to appear in these 

 skate embryos scarcely show signs of this specialisation, and those in the 

 branchial region are little more advanced. Of course the most advanced 

 cells are not at the posterior end, for this specialisation needs time. 



