MUSEUM OF COMPARATIVE ZOOLOGY. 59 



ance which he has figured must have had a basis iu optically different 

 portions of that envelope. According as the imbibition of water has 

 proceeded less or more, this marking might be more or less conspicuous. 

 From a comparison of the figures by Calberla with those by the last 

 mentioned authors, I should think that Calberla's outer layer of the 

 zona by no means corresponded with the outer layer of Kupffer und 

 Benecke, and that the latter, being very thin, had been overlooked by 

 Calberla. 



The micropyle of Petromyzon, though sought for by Schultze ('56) 

 and A. Miiller ('64) was not found by them. 



Owsjannikow ('70 a , p. 184), who discovered it, says that it is very 

 small, but that it remains visible for several days after fertilization. In 

 mature eggs it occupies a position over the eccentric nucleus. 



Calberla ('78, pp. 439, 440) has given a careful description of the 

 micropyle, which, he says, agrees in all essential particulars with that of 

 osseous fishes. His account is substantially as follows. At one pole 

 of the elongated egg its membrane is thickened, and bulges out, much 

 as though a shallow watch-glass — with shorter radius of curvature than 

 the rest of the egg membrane — had been set into one end of the mem- 

 brane. Radial sections which pass through the centre of the elevated 

 portion of the membrane show that in the middle of it there is a very 

 flat saucer-shaped depression, the centre of which is further depressed 

 into a funnel. From the narrow end of the funnel a canal is continued 

 through the membrane, and opens on its inner surface with a slight 

 flaring. A little below its middle the canal exhibits a spindle-shaped 

 enlargement, which is shown in Calberla's Taf. XXVII. Figs. 2 and 3. 



The views held by Kupffer und Benecke ('78, pp. 9-15) regarding 

 the nature of the micropyle are not easily summarized. They are based 

 on close observations of the deportment of the egg and spermatozoa at 

 the time of fertilization, but do not appear to have been corroborated 

 by sections of the egg membranes. 



In the region of the watch-glass segment of the membrane described 

 by Calberla, the mucilaginous envelope outside the membranes is want- 

 ing, and in its place is a hyaline dome (A. Midler's " Flocke ") composed 

 of a substance which, unlike the mucilaginous layer, is permeable for 

 spermatozoa. Usually only one spermatozoon passes through the inner 

 and outer egg membranes and reaches the yolk ; but the place of its 

 passage is by no means always the centre of the watch-glass area. It 

 was such only six times out of fifty. The passage may occur even near 



