THE FERTILIZATION-PROCESS 



of thread-like projections of granule-free cytoplasm into 

 the space between the vitelline membrane and the egg- 

 plasma. Later by the anastomosis of the free ends of these 

 threads a very thin sheath forms; the threads and their 

 enclosing sheath constitute the hyaline plasma-layer. 

 Fifteen minutes after fertilization (temperature of the sea- 

 water around 2i°C.) the hyaline plasma-layer stands out 

 very sharply even under low power of the microscope. 

 During this same period the sperm-head evolves as the 

 sperm-nucleus with attendant cytoplasmic changes. 



Although I have found it easy to follow the history of 

 the sperm-head in the egg of Arbacia, I prefer at this point 

 to base the following description on the egg of the flat sea- 

 urchin or "sand-dollar," Echinarachnius. This I do 

 because the &^g of Echinarachnius being larger and not so 

 highly colored as that of Arhacia lends itself readily to 

 exact observation in the living state. For both, except 

 for minor variations, the process is the same. Because of 

 the significance ascribed to the middle-piece of the sperma- 

 tozoon in echinids by Boveri's theory of fertilization, it is 

 necessary to take up the history of the middle-piece in 

 some detail. 



During the stages of sperm-attachment and penetration, 

 the middle-piece reveals the same structure and position 

 found in the free-swimming spermatozoon. That is, it is 

 closely fitted to the basal end of the sperm-head and shows 

 prominently a bipartite granule. In fixed preparations 

 treated with a dye, haematoxylin, the nucleus of the sperm- 

 atozoon stains bluish gray; the middle-piece, the outer 

 limits of which are continuous with the nuclear membrane, 

 is also gray. The middle-piece granule is seen as a sharply 

 defined black body, lying — as in the free-swimming sperma- 

 tozoon — in various positions in the middle-piece. Though 

 in some views it shows up as a continuous horse-shoe shaped 

 body and in others as two rods, it is in reality a body com- 



171 



