ON THE DEVELOPMENT OF TUBULIPORA. 135 



tory vesicles and a few cellular contents. The vestibule has 

 well-defined walls lying in the body-cavity of the ovicell. The 

 terminal membrane has become distinct; it closes the body- 

 cavity at its distal end, and is somewhat thickened, except at 

 the middle. 



Fig. 21 represents an ovicell of T. liliacea in stage D. 

 Although I have only one series of sections of this species in 

 the stage in question, its characters correspond so closely with 

 those of the succeeding stage (fig. 20) that I regard it as a 

 normal ovicell. If this be admitted, it follows that there is a 

 marked diflference between T. liliacea and the other two 

 species (T. plumosa and T. phalange a), its embryophore 

 being much longer than anything which occurs in them, and 

 being in fact as well developed as that of Lichenopora. 

 Fig. 21 is drawn to the same scale as figs. 16 and 18, so 

 that comparison is easy. 



The other features of the section resemble those found in 

 the other species. The distal part of the ovicell is still solid, 

 and contains but few excretory vesicles (not seen in this 

 section) ; the vestibule can be made out, and the brown body 

 has a thick investment continuous proximally with the embryo- 

 phore, and having distally a split leading to the vestibule. 

 The existence of the nutritive tissue shows that the ovicell is 

 at the end of stage D, a conclusion which is also indicated by 

 the length of the embryophore. 



The general features of stage D are thus as follows : — The 

 egg-follicle becomes replaced by what may be termed the 

 embryonic follicle, or simply the follicle. This becomes sepa- 

 rated from the brown body by intervening cells, which are 

 specially well developed in T. liliacea. The brown body, at 

 first without any distinct cellular investment, becomes sur- 

 rounded by a mass of cells which mark it out sharply from 

 the other parts of the ovicell ; and the proximal part of this 

 investment gives rise to a special " nutritive tissue." The 

 vestibule makes its appearance, at first surrounded by a nearly 

 solid mass of cells containing excretory vesicles, into which a 

 median terminal invagination of the ectoderm projects. The 



