BANCROFT: OVOGENESIS IN DISTAPLIA OCCIDENTALIS. 89 



over, in the latest ovarian stages, where the follicle is even thinner than 

 in Figure 30, they are still less apparent. But later developments show 

 conclusively that we here have to do with the nuclei of the inner follic- 

 ular layer; for when the egg has left the ovary it is covered by only a 

 thin pavement epithelium with pale nuclei at rather distant intervals 

 (Plate 5, Fig. 31, e'th. fol. i.), while all the darker nuclei of the outer 

 follicle remain behind in the corpus luteum (Plate 1, Fig. 5 ; Plate 6, 

 Fig. 47). This inner epithelium persists, and covers the embryo during 

 the whole of its life within the colony, and becomes so stretched in the 

 later stages that even the nuclei are obliterated. It is thus seen that, 

 contrary to what DavidofF believed (p. 138), there is in Distaplia a 

 splitting of the secondary follicle into an outer and an inner epithelium, 

 which is actually accomplished only when the Qgg leaves the ovary, 

 though the two layers are probably, and their nuclei certainly, differ- 

 entiated some time before. Thus, in Distaplia, the results of van 

 Beneden et Julin on the origin and fate of these two epithelia are com- 

 pletely confirmed. 



5. Outer Follicular Epithelium and Corpus Luteum. 



As has already been said, one of the distinguishing features of this 

 layer is the comparatively deep stain that its nuclei take. Another 

 characteristic, which is just beginning to appear in Figure 30 (Plate 5), 

 but is much more distinct later, is that part of the cytoplasm also takes 

 a very deep stain. Tangential sections through the follicle of ova that 

 have reached the maximum size, about 300 ^, show this stained cyto- 

 plasm situated in irregular patches around a central nucleus. In sec- 

 tions of the newly formed corpus luteum (Plate 6, Fig. 47) this 

 cytoplasm comes out very distinctly. 



The corpora lutea (Fig. 5) are often conspicuous features of the 

 Distaplia ovary, as they are at first about 80 /x in diameter, and rem- 

 nants of them persist until an embryo of forty or fifty cells is developed 

 from the ovum formerly contained within them. When first formed, 

 this structure is a thick walled vesicle with a rather small cavity open- 

 ing into the lumen of the ovary. Almost the whole thickness of the 

 wall is composed of the cells of the outer follicle, which are pressed 

 together and elongated radially. Figure 47, which represents part of 

 the wall of a corpus luteum which the hind end of the ovum has just 

 left, shows most of the deeply stained cytoplasm of each cell located 

 between its nucleus and the periphery of the organ. Peripherally, this 



VOL. XXXV. — NO. 4, 3 



