THE FERTILIZATION-PROCESS 



Within the astrosphere the middle-piece granule is bril- 

 liantly revealed — a black eccentrically placed body, in 

 sharp contrast to the clear substance immediately around 

 it and the grayish blue cytoplasmic constituents farther 

 beyond. The granule has never been found at the centre 

 of the astrosphere. This is easily established in those 

 stages, like Fig. 29/^, in which the astrosphere is well 

 developed. 



During the ensuing stages of sperm-penetration up to 

 the stage of apposition of the nuclei, the sperm-nucleus 

 increases in size. The greater the distance, and therefore 

 the time, which the sperm-nucleus must travel before reach- 

 ing the egg-nucleus, the greater is its increase in size. This 

 distance, of course, depends upon the site of sperm entry, 

 which may be at any point on the egg-surface, with refer- 

 ence to the location of the egg-nucleus, which has no fixed 

 position. It is therefore very easy to obtain a large number 

 of stages during penetration. 



During these stages I have found a behavior of the mid- 

 dle-piece granule which differs from that described by 

 Meves for the egg of Parechinus — a difference which may 

 be due to technique, though I doubt that since the his- 

 tory of the granule in fertilized eggs of Arbacia fixed as 

 those of Echinarachnius is like that of Parechmus. This 

 difference is that the middle-piece granule in the egg of 

 Echiiiar achnuis separates from each of its free limbs a more 

 minute granule. Thus each limb of the original bipartite 

 granule in turn becomes bipartite. These two new forma- 

 tions move away from the parent granule, each maintaining 

 connection by means of a delicate thread. Figs. {2gb), 

 (29c) and show this formation. 



As the sperm-nucleus increases in size, its basal thread, 

 at the tip of which is the middle-piece granule, becomes 

 longer, an indication of the ductility of the sperm-nucleus 

 during these stages. In form, therefore, it resembles a 



^73 



