No. 2.] THE SEA-URCHIN EGG. 447 



the entrance-cone, stain intensely blue in iron-hsematoxylin, 

 while the clearer substance in which they lie takes the haema- 

 toxylin very slightly, but is colored red by Congo red. 



The first astral rays become visible when the sperm-head has 

 rotated about 90 degrees, though in rare cases they may appear 

 before the rotation has begun. In its first stages the aster 

 seems to be nothing more than a slight radial arrangement of 







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B 



Fig. I. — Three successive stages in the formation of the sperm-aster. From sections. 



A . Sperm-head soon after its entrance, showing the entrance-funnel, the cyto-reticulum com- 

 posed of minute microsomes, the rotating sperm-nucleus and the spherical middle-piece at 

 its base. 



B. Later stage, showing the entrance cone and funnel, the advancing rotation and the astral 

 rays in process of formation out of the reticulum. 



C. The sperm nucleus and aster shown in Prototype 2, drawn on a larger scale, showing the 

 relation of the rays to the reticulum. (3 minutes.) 



the mesh-work about the middle-piece (Text-fig. I, B). The 

 latter may from this time forwards be called the central mass. 

 As the aster increases (Phototype 2, Text-fig. I, C) the rays 

 are clearly seen to be differentiated out of the reticulum, 

 passing out into its meshes, where their outer ends become, as 

 it were, lost in its substance. Some of the rays extend out- 

 ward into the entrance-funnel, and may be traced in some 

 cases actually into the entrance-cone. 



At a first examination the rays have the appearance of 

 continuous fibres ; when closely examined, however, in the 



