No. I.] 



THE ECHINODERM EGG. 



,27 



In many cases, if not in all, it is the aster that first comes in 

 contact with the egg-nucleus, against which it becomes closely 

 pressed (Fig. 2, H). The sperm-nucleus, as it follows the 

 aster, loses its lance-shaped outline, becomes oval, swells some- 



FiG. 2. Entrance and further history of the spermatozoon up to its conjuga- 

 tion with the egg-nucleus in Toxopneustes (this and the following figures from 

 camera drawings of sections after fixation with sublimate-acetic, stained on the 

 side with iron-haematoxylin and Congo red). A, free sperm-head, with granular 

 middle-piece ; B, C, D, E, F, successive stages in the penetration and rotation of 

 the sperm-head, and the development of the sperm-aster ; G, pronuclei just before 

 union; H^ union of the pronuclei and growth of the sperm-aster; two sperm- 

 heads attached to the egg-periphery. 



what, and finally comes into contact with the egg-nucleus, 

 beside the archoplasm-mass (Fig. 3, A). The central mass 

 now stains bright red with acid-fuchsin, has a definite though 

 irregular outline, and a slightly vacuolated appearance. The 

 archoplasm-mass rapidly enlarges and is flattened against 



