492 BULLETIN OF THE 



after fertilization. This account seems to need confirmation, for the 

 greatest care must be exercised to insure the observer against the possi- 

 bihty of having before him eggs that are no longer in an active living 

 condition. Hoffmann has observed clear protoplasmic elevations on the 

 surface of the yolk, such as Butschli has figured for Nephelis, but since 

 the same phenomenon is seen on eggs that have certainly not been 

 brought within the influence of spermatozoa, he thinks it cannot be 

 that it has resulted from a metamorphosed spermatozoon. It may be 

 considered as certain, he holds, that the penetration of the spermatozoa 

 and the subsequent lively motion of the yolk granules induce the grad- 

 ual disappearance and probably the complete elimination of the nucleus 

 (germinative vesicle), for on artificially fecundated eggs so many sper- 

 matozoa are sometimes attached to the yolk as to put it in rotation, and 

 in such cases the nucleus is ejected in its full size an hour after fer- 

 tilization, whereas normally, when only a few spermatozoa are attached 

 to the egg, the two small polar globules do not appear until two hours 

 after fertilization. (!) 



In the case of Clepsine, Hoffmann ('77", p. 34, Taf. III. Fig. 5) 

 seems to have seen something of the asters of the male and female pro- 

 nuclei. He has figured and described in a section of an egg prepared 

 several hours after extrusion, the existence of two places in the coarsely 

 granular yolk which are filled with a finely granular substance, radially 

 arranged about the centre of each spot. 



In Toxopneustes variegatus Selenka ('78) has observed the penetra- 

 tion of spermatozoa of which he gives this account. Usually only one 

 spermatozoon succeeds after a long boring motion with its pointed head 

 in passing through the jelly-like zone which envelopes the q^^^. As soon 

 as it gets near the yolk it is suddenly enabled to swim rapidly and 

 easily in all directions over the plasma mantle which envelops the yolk 

 (see p. 458). The passage it has made through the zona remains open 

 and is often traversed by both inward- and outward-going spermatozoa. 

 The spermatozoon usually penetrates the yolk at the " Dotterhiigel," 

 and causes a distinct agitation in the surrounding parts by its boring 

 motion. A tufted* mass of clear substance at once collects around the 

 head of the spermatozoon from the clear mantle of protoplasm which 

 surrounds the egg. As it penetrates deeper into the yolk this tuft of 

 protoplasm sinks with it ; thus forming a depression from the middle 

 of which the "tail," which soon becomes motionless, projects as a fine 

 filament. When it has penetrated about a quarter of the way to the 

 centre the automatic motion almost instantly ceases, and within half a. 



