ENTRANCE OF THE SPERMATOZOON, ETC. 



13 



II. ENTRANCE OF THE SPERMATOZOON; ROTATION OF THE SPERM-HEAD 



SPERM-ASTER. 



ORIGIN OF THE 



M - 

 N 



\ 



B 



'm^.-. 



c 





In many animals the spermatozoon enters the egg at a predetermined point, and if the egg is surrounded by a 

 membrane (as, for example, in insects), there may be at that point a special opening or micropyle through which the 

 spermatozoon enters. In the sea-urchin the spermatozoon may enter the naked egg at any point. Its lance-shaped 

 head (consisting of nucleus and middle-piece) first comes into contact with the 

 egg periphery by its tip (Te.\t-fig. II. B). Instantly afterwards a rush of the 

 neighbouring egg-protoplasm takes place towards the point of contact, forming a 

 conical prominence, the aitj^aiicc-couc, into which the sperm-head passes. The 

 cone afterwards assumes a ragged flame-shape (Text-fig. II. C, D), and finally 

 disappears a few minutes after the entrance. At the moment the cone is formed 

 a delicate membrane, the vitelline membrane (v. Text-fig. II.) is thrown off by the 

 egg. This carries with it the tail of the spermatozoon, attached to its outer 

 surface (Text-fig. II. D, E), so that only the head (nucleus and middle-piece) enters 

 the egg.' The vitelline membrane, which entirely surrounds the egg, prevents 

 the entrance of other spermatozoa, and thus avoids polyspermy. The Hertwig 

 brothers have shown that if eggs (of the sea-urchin) be treated with weak solutions 

 of chloral hydrate and other substances, the formation of the vitelline is prevented 

 or delayed, and such eggs are almost invariably polyspermic. 



In the living egg the sperm-head usually disappears from view after its 

 entrance, but in its place a small star, the spcrvi-astcr (Text-fio-. II. G s) appears fig. ix. — Entrance and rotation of the sperm-head. 



^ \ & ' /' rtr Origin of the sperm-aster about the middle-piece. 



two to three minutes afterwards near the entrance-cone. Sections of preserved eo-o-s 

 suitably stained show, however, that the sperm-head persists, and that it rotates 

 within the egg immediately after its entrance, so that the base is turned inwards 

 (Text-fig. IX.). At the same time the middle-piece becomes the centre of a 

 beautiful star-shaped structure, (the sperm-aster) the rays of which arise from the 

 general reticulum. This aster, which afterwards increases enormously in size, is 

 destined to divide into two halves, which, with the spindle developed between 

 them, form the amphiaster of tlie first cleavage. 





Fig. IX. 



sperm-aster about the middle-piece. 

 From sections. Enlarged 2000 diameters. 



A. The sperm-head before entrance into the egg, as 



it appears after double-staining with hcematoxy- 

 lin and Congo red. N, nucleus (deep blue- 

 black); M, middle-piece (pale red). 



B. Immediately after entrance, showing the entrance- 



cone. 



C. Beginning of the rotation. 



D. Sperm-nucleus rotated 90°; aster developing 



about a central mass lying at the base of the 

 nucleus and derived from the middle-piece. 



E. F. Completion of the rotation; growth of the 



astral rays. The last figure is 3| minutes after 

 the rirst entrance. 



Plate II. Phototype 5. 



The First Entrance of the Spermatozoon. 



In this figure the sperm-head may be seen immediately after its entrance as a black, lance-shaped body at the 

 lower pole of the egg opposite to the egg-nucleus. Another sperm-head, which has not entered, is seen at one side, 

 lymg at the periphery of the egg. The sperm-head, as thus seen, represents only the nucleus of the spermatozoon. 

 The middle-piece, which lies at the base of the nucleus, is not distinctly visible, since it is stained of the same 



[n many cases, however, — e. g., in the a.xolotl, the earthworm, the butterfly,— the tail also enters the egg 



