134 SIDNEY F. HARMER. 



being continuous with the vestibule. The terminal membrane 

 is deeply invaginated raedianly ; and this is in fact a general 

 feature of ovicells in this stage. In sections which are not 

 median the invagination may not be seen ; and in some cases^ 

 as in fig. 18, it is obscured by the great development of the 

 excretory vesicles. The existence of the invagination may, 

 however, be looked on as the general rule ; and it is probable 

 that the vestibule is really continuous with, and has been 

 developed by invagination from, the terminal invagination, 

 though it is not easy to demonstrate this continuity. The 

 wall of the vestibule, during the beginning of stage D, is 

 closely surrounded by the other tissues of the solid distal half 

 of the embryophore, so that the whole of its limits cannot, in 

 most cases, be made out with certainty. Excretory vesicles 

 were but slightly developed in this ovicell (fig. 16). 



Fig. 17 is a somewhat oblique longitudinal section of the 

 distal half of an ovicell in about the same stage as fig. 16. 

 Part of the median invagination of the terminal membrane is 

 seen as a slit-like cavity. The section shows the investment of 

 the brown body, and the way in which it is connected with 

 the vestibule, the junction being constricted like the neck of 

 a flask. This is a normal arrangement, as is also (in T. 

 phalangea at least) the oblique position of the neck of the 

 flask-like connection. 



Fig. 24 (PI. 10) represents an ovicell of T, plumosa in 

 an unusual condition. While the embryo and the proximal 

 half of the embryophore are in stage D, the remainder of the 

 ovicell has the form characteristic of stage E. 



It can hardly be doubted, from the sections of stage D, that 

 the ovicell at this time does not differ, or hardly differs, 

 externally from a zooecium. The young ovicell lies completely 

 in series with the ordinary zooecia, and there is no indication 

 of a distal dilatation of its cavity. 



In Fig. 24, however, the distal end of the ovicell is clearly 

 dilated, and this expansion has been accompanied by the 

 vacuolatiou of the previously solid or semi-solid distal half of 

 the ovicell, which now contains a spacious cavity, with excre- 



