I GROWTH OF THE GERM- CELLS 29 



the head and tail are both directed to the outside. The 

 entrance is now completed. It is effected by the formation, 

 under the influence of the acrosome, of a substance which, by 

 virtue apparently of its capillary properties, moves into the 

 egg and carries the sperm bodily with it. The sperm is in all 

 cases rotated through 180 in the process, so that the middle- 

 piece is inwardly directed. 



The sides of the entrance-funnel are lined, in the Axolotl 

 and Frog egg, by pigment dragged in from the surface. This 

 pigment remains as a streak marking the position of the funnel 

 when the latter has disappeared. The entrance-funnel is 

 known in these eg-gs as the first part of the sperm-path. 



A clear, yolk-free area is now developed round the middle- 

 piece or centrosome : this is the sperm-sphere, and from its 

 periphery radiations soon arise and pass out between the 

 yolk-granules, the sperm-aster. The formation of the sperm- 

 sphere is probably also due to the withdrawal of water from 

 the cytoplasm by the centrosome. The centrosome is in any 

 case used up during the process, and entirely disappears. The 

 sperm-head, now detached from the tail, quickly shortens and 

 thickens to become the sperm-nucleus, or male pronucleus. 



3. The definitive centrosome is now developed. It arises 

 in the Axolotl from the sperm-nucleus. The nuclear mem- 

 brane opens on the inside, and there emerges a dense rounded 

 body, or more probably there emerges something (? nucleic acid) 

 which produces this body by precipitation of the proteins of 

 the egg. However formed, this body is the definitive centro- 

 some. The sperm-nucleus is now moving along what is known 

 as the second part of its path to meet the female pronucleus. 

 The latter has moved from its position at the animal pole 

 towards the centre of the egg, usually but not necessarily 

 along the egg-axis. Thus the second part of the path may or 

 may not lie in the same meridional plane as the first part 

 (entrance-funnel), and in the former case it usually makes an 

 angle with the first part, since it is directed to a point approxi- 

 mately in the axis and at a fixed distance from the animal 

 pole, while the point of entrance of the spermatozoon may be 

 anywhere in the animal hemisphere (see below). 



