MUSEUM OF COMPARATIVE ZOOLOGY. 107 



of granules and its thread-like continuation the author regards as a pro- 

 duct of the granulosa cells. But at present there are no data, he says, 

 either teleological or phylogenetic, which can explain the remarkable 

 structure of the external micropyle. 



Ryder ('86, p. 30, Plate VII. Fig. 35), who has demonstrated the 

 existence of the micropyle in the eggs of several fishes, speaks of the 

 passage through the capsular membrane in Perca Americana as "a 

 wider pore-canal which leads to the micropyle." It is evident from 

 Eigenmann's ('90) account and from his figures that this statement is 

 inaccurate. 



Cunningham ('86 a , pp. 59, 61-63, etc., Figs. 2-4, 12) has shown much 

 more satisfactorily than W. Miiller (75) did, that in Myxine glutinosa 

 the follicular epithelium plays an important part in the formation of the 

 micropyle. Of an egg 16 mm. in length he says : " At the exact pole of 

 the egg there is a differentiated portion of epithelium, where a prolife- 

 ration of the latter has taken place. This portion is composed of poly- 

 gonal cells, which are little or not at all elongated, and towards the egg 

 it runs out into a thin cylindrical process which penetrates the next layer 

 [zona radiata] as shown at e. p. [Fig. 2]. . . . This process penetrates 

 the vitelline membrane [zona radiata], occupying a tubular cavity in the 

 latter, and passing through it to form a hemispherical projection on its in- 

 ner surface. . . . This cellular projection is covered by a thin membrane 

 continuous with the vitelline membrane, and is not in immediate con- 

 tact with the germinal disk. . . . There is thus at one pole of the nearly 

 ripe ovum a tubular canal extending through the chorion [zona radiata], 

 but not open internally, filled up by a cylinder of cells projecting from 

 the follicular epithelium. ... It is evident," the author adds, w that this 

 aperture is to form the micropyle in the ripe ovum. ... It is probable 

 that careful investigation would show that in all Teleosteans whose ova 

 possess a micropyle that structure is produced by a projection of cells 

 from the follicular epithelium." Cunningham also believes it " at least 

 possible that in all vertebrates the micropyle will be found on investiga- 

 tion to be produced in the same way as in Myxine, namely, by the 

 growth of a cellular process from the follicular epithelium towards the 

 vitellus while the vitelline membrane is being formed." 



In an egg 20 mm. in length " the proliferation and differentiation of 

 cells at the pole in the follicular epithelium have disappeared, but the 

 cylinder of cells, though reduced in size, still remains in the micropjde, 

 and is evidently destined to keep the latter open until the maturation 

 of the ovum is complete." This egg was from material obtained in 



