28 INTRODUCTORY I 



different. Here the membrane is preformed the vitelline 

 membrane. Immediately below it there is a peripheral layer 

 of radial alveoli filled with a gelatinous material. Internal 

 to this is the cytoplasm with yolk-spherules and oil globules, 

 and of course the nucleus. Immediately upon contact of a 

 spermatozoon with the surface of the vitelline membrane the 

 gelatinous material of the peripheral alveoli is extruded as 

 a broad zone of jelly. Sea-water enters and occupies the 

 alveoli, whose walls remain as strands or lamellae crossing 

 the peri vitelline space between the membrane and the ovum. 

 The perivitelline space is therefore, as in the Sea-urchin, in 

 a sense a part of the ovum. 



In the Frog, a fluid is excreted, and this pushes away from 

 the ovum the vitelline membrane, which remains adherent to 

 the jelly. The ovum is now free to turn in the fluid, and 

 does so until the axis assumes a vertical position with the 

 heavy white pole below. This appears about half an hour 

 after insemination. 



2. As soon as the acrosome of the spermatozoon has 

 perforated the surface-layer of the egg-cytoplasm there 

 begins to collect around it a substance, hyaline in appearance 

 (in the Axolotl) and apparently of a more watery consistence 

 than the rest of the cytoplasm. It increases quickly in amount 

 and assumes the form of a cone, the apex of which is directed 

 towards the interior of the egg. It is known as the entrance 

 funnel (Fig. 8). Its base usually projects slightly from the 

 surface : this is the entrance cone (erroneously termed cone of 

 attraction). The entrance-funnel, still increasing, moves on 

 into the interior of the ovum, and the spermatozoon is carried 

 impassively with it. In the Axolotl (and others) the tail is 

 also carried in, but in some cases (Mus, Nereis, for instance) it 

 remains outside. Even when taken into the egg it degenerates : 

 it has no part to play in the later stages of fertilization. The 

 acrosome now seems to get caught in the side of the entrance- 

 funnel and, the inward streaming movement of the latter 

 continuing, the head of the spermatozoon is driven on and 

 rotated through 180, so that the neck (containing the centro- 

 some) now lies at the inner end of the entrance-funnel, while 



