ENTRANCE OF THE SPERMATOZOON ETC. 



15 



shown, extending from the base of the sperm-nucleus outwards to the periphery, where the entrance-cone appears 

 (not quite in focus). The centre of the sperm-aster, which may now be called the centrosphere or centrosome 

 appears as a finely granular mass (see Text-fig. X. A and B), lying at the apex of the funnel and evidently 

 derived from the middle-piece. The rays proceeding from it run out at their tips into rows of granules indistin- 

 guishable from the microsomes of the general reticulum, with which they are perfectly continuous. These is reason 

 to believe, as explained beyond, that the astral rays do not properly grow forth from the central mass, as they seem to 

 do, but arise through a radial rearrangement of the pre-existing reticulum under the influence of the centrosome 

 to which they converge. 





-••■cT'T^'N'/. '- 





•^\I^.,■^•^ 







■■~4 "" y-< 





B 



Fig. X. 



Fig. X. — Structure and growth of the sperm-aster; 4000 diameters. 

 A Very early stage (i^ minutes). The middle-piece (now the centrosome) forms a distinct rounded body lying at the base of the sperm-nucleus. A number of short astral ravs are 

 forming about it. They may be traceil out at their tips directly into the general reticulum. 



B. Later stage (3 minutes). The sperm-head has moved further inwards, its rays are more numerous and extend further out into the cytoplasm. .\s the figure shows, the rays arise by 

 the direct transformation of the reticulum. 



In some cases the central mass of the sperm-aster is described as containing an extremely minute deeply staining 

 bod)', which has been regarded as representing the true centrosome at this stage (see p. 20). Such a body cannot be seen 

 in Toxopneustcs, and the evidence indicates that the entire central mass (i.e., the substance of the original middle-piece) is 

 here the centrosome.' 



Plate III. Phototype 10. 



The Rotation Completed. Sperm-aster lying beside the Sperm-nnelciis. 



In this somewhat exceptional specimen the sperm-nucleus has completely turned, so that its base is directed 

 inwards, while the aster lies quite at one side. [As a rule the aster remains exactly at the base of the nucleus.] The 

 funnel and entrance-cone are faintly shown, also the egg-nucleus, slightly out of focus. A single lance-shaped sperm- 

 head shown outside the eesr. 



^ See Wilson, Archoplasm, Centrosome, and Chromatin in the Sea-urchin Egg. Journ. Alorph., Aug., 1895. 



