132 SIDNEY P. HARMER. 



Stage D. — Definitive Formation of the Embryophore. 



The appearance of the ovicell in this stage is very charac- 

 teristiCj and from the frequency with whicli it occurs in my 

 sections I conclude that the stage is of relatively long dura- 

 tion. 1 have examined about seventeen ovicells in this stage 

 of T. plumosa, twenty-seven of T. phalangea^ and one of 

 T. liliacea. It appears to me that the embryophore of the 

 first species is normally distinctly larger than that of the 

 second, while that of the third species diff'ers from the other 

 two in its great length. 



Fig. 15 shows an embryophore of T. plumosa at the be- 

 ginning of this stage. The brown body is fully formed, but 

 appears young, and it is not yet surrounded by any very 

 definite investment of cells. The embryo has two obvious 

 blastomeres, and the follicle-cavity in which they lie is in the 

 immediate neighbourhood of the brown body. The cord of 

 cells running proximally from the follicle is the shrunken 

 remains of the somatic mesoderm of the fertile polypide. The 

 terminal membrane is thickened, and its staining properties 

 indicate that it is in a state of active growth. It is continuous 

 internally with a mass of cells which extend from it to the 

 brown body. This mass contains four excretory vesicles, 

 though a larger number were visible in some of the other sec- 

 tions of the same ovicell. 



The length of the embryophore, from the proximal end of 

 the follicle to the distal tip of the terminal membrane, is 

 200^. 



Fig. 14 (T. phalangea) illustrates the condition in which 

 the terminal membrane is usually found in this stage. The 

 distal end of the embryophore projects into the cavity of the 

 young ovicell as a knob, whicli appears to be quite free from 

 the wall of the ovicell. The brown body is fully formed, and 

 has a more compact appearance than that of the former speci- 

 men. 



Fig. 18 is a slightly older embryophore of T. plumosa, 

 from a colony in which excretory vesicles were specially 



