MUSEUM OF COMPARATIVE ZOOLOGY. 501 



the same time the place where the pedicellate egg^ up to a short time before 

 its liberation, was connected ivith the ovarian wall, and that therefore the 

 mantle is here softer." The fact that the absence of a membrane allows 

 the polar globules to lose entirely their connection with the yolk, makes 

 such a mistaken identification as I have suggested extremely easy, or 

 at least would make the proof of the accuracy of the identification at- 

 tainable only by continuous observation of the same egg. If Selenka's 

 grounds for disconnecting the polar globules and the position of the first 

 cleavage plane are insufficient, those for connecting the position of the 

 plane with that of the radius along which the spermatozoon penetrates 

 are limited to the statement of the fact, and therefore lie beyond the 

 reach of criticism.* 



If those of Selenka's conclusions which I have criticised should prove 

 to be untenable, the objections which he urges against the use of " for- 

 mative pole " and " polar globules " in Toxopneustes would be no longer 

 valid. 



While KuPFFER und Benecke ('78) confirm the most interesting point 

 of Calberla's observations on Petromyzon, — the penetration of a sper- 

 matozoon into the yolk, — they differ in many points of importance. 

 The micropyle is not always situated at the summit of the watch-glass- 

 shaped portion of the eg^ membrane. Only the inner layer of the egg 

 membrane is provided with pore canals, not both layers, as Calberla 

 claims. The cap of clear protoplasm which lies immediately under the 

 watch-glass area of the membrane is not continued as a thinner layer 

 around the whole yolk, but has about the same extent as does the watch- 

 glass elevation. Only those spermatozoa which attain the hyaline dome 

 (A. Mtiller's " Flocke ") surmounting the watch-glass segment of the egg 

 membrane are of concern in the act of fertilization. All such at once 

 assume a direction radial to the "watch-glass," and as soon as the first 

 has reached this dome the yolk begins to withdraw from the egg mem- 

 brane, leaving an annular space corresponding with the rim of the watch- 

 glass. The contraction of the yolk therefore results from the influence 

 of the spermatozoa at a distance. This retraction is more lively if 

 several spermatozoa enter the dome instead of one. The further retrac- 



* In what sense the spermatozoon radius is held to be "determining" can pos- 

 sibly be inferredirom an examination of his figures. From Figs, 16 and 17 it ap- 

 pears that the first cleavage plane (if perpendicular to the long axis of the nucleus) 

 neither coincides with nor is perpendicular to this radius. Therefore, the only con- 

 clusion that seems possible is that the position of the segmentation plane is held to 

 be dependent on two factors, — one the direction of the spermatozoon radius, and the 

 other the extent of the angular rotation of the fusing pronuclei. 



