i 



BANCROFT: OVOGENESIS IN DISTAPLIA OCCIDENTALIS. 79 



the pouch is at about the level of the stomach, the first ovum enters it; 

 when the last ovum has entered, its end is about at the level of the 

 posterior extremity of the zooid. 



None of the pouches containing the older embryos are ever found 

 connected to zooids, and as.no adult zooids are ever encountered with- 

 out pouches, it is evident that the zooids from which the older pouches 

 arose have degenerated, and that those found in the colony along with 

 these older pouches belong to a subsequent generation. As a matter of 

 fact, degenerating zooids are not infrequently encountered, and in close 

 proximity to them are seen the pouches with which they were probably 

 connected. Most of the growth of the embryos takes place after the 

 degeneration of their mother; accordingly the pouch is much enlarged, 

 becoming even longer than the adult zooid was, and the narrow ante- 

 rior stalk apparently swells up to the size of the pouch proper. The 

 whole structure migrates through the test until its anterior end finally 

 comes into close connection with one of the common cloacal cavities, 

 through which the fully developed larvae reach the exterior. 



IV. Envelopes of the Ovum. 



So many summaries of this subject have been published, and such a 

 good one by its most recent investigator, Floderus, that a detailed 

 discussion of the literature would be superfluous. In fact the whole 

 subject has been left in a very satisfactory condition by this author, 

 who was the first to discuss the evidence for the formation of the follicle 

 cells and test cells, both from within the ovum and from sources which 

 are external to it. Accordingly, much that I have determined is con- 

 firmatory of his results, and of those of earlier investigators. How- 

 ever, as it was from the genus Distaplia that Davidoff ('89) drew what 

 is probably the strongest evidence for the intraovular origin of the test 

 cells, it seems worth while to discuss the conditions in this genus. 



1. Primitive Follicular Epithelium. 



Concerning the development of the primitive follicular epithelium 

 (van Beneden et Julin, '85, p. 357), I can confirm the account of 

 Moderns and all the other investigators of the subject since van Beneden 

 et Julin's time. The epithelium is formed from the primordial follicb 

 cells found in close proximity to the oogonia in the earliest stages 



