62 J. T. CUNNINGHAM. 



nearly ripe ovum a tubular canal extending througli the chorion, 

 but not open internally^ filled up by a cylinder of cells project- 

 ing from the follicular epithelium. It is evident, on considera- 

 tion of the above facts, that this aperture is to form the micro- 

 pyle in the ripe ovum, and we have here, as will be explained 

 more fully below, the explanation of the process of fertilisation 

 in the ovum of Myxine. It is very improbable that sperma- 

 tozoa could penetrate such a thick dense capsule as is formed 

 by the vitelline membrane in the ripe ovum, and thus the 

 presence of a micropyle is necessary. Another point of some 

 interest in this connection is that we have here for the first 

 time the complete history of the origin and formation of the 

 micropyle in a vertebrate ovum. A micropyle is known to 

 exist in many Teleostean ova, but little investigation has been 

 made as to how the structure is produced in the course of the 

 ovarian development of the ovum. 



In a paper which has recently come into my hands ("Studien 

 iiberdasEi," 'Memoires del'Acad. Imp. St. Peters.,' 1885), Ph. 

 Owsjannikow describes some observations on the micropyle in 

 the ovum of Osmerus eperlanus, the comparison of which 

 with my description and figures is very interesting. The eggs 

 studied by Owsjannikow were not naturally shed, but taken 

 when almost ripe from the ovary. In this condition the layer 

 of follicular epithelium, or granulosa, as the Russian investi- 

 gator calls it, is frequently found attached to the egg mem- 

 brane. The egg membrane consists of two layers, called zona 

 radiata externa and zona radiata interna. A micropyle 

 pierces both of these layers, and is expanded like a funnel 

 externally. A conical projection from the granulosa was seen 

 to extend into the micropyle. This conical projection was not 

 solid as in the case of Myxine, but hollow, forming a lining to 

 the micropyle. A thin thread was seen to extend from the end 

 of the funnel-shaped opening in the external zona, through the 

 internal micropyle, which was a narrow canal in the internal 

 zona. A row of granules was also seen to extend from the 

 inner end of the internal micropyle into the vitellus. Con- 

 cerning the thread and row of granules Owsjannikow gives no 



