164 MORPHOLOGY OF FERTILIZATION 



even continued to progress inward. If, as Lillie described it, the 

 "fertilization cone" is a cell organ which moves a considerable 

 distance into the egg, then one might speculate that the mecha- 

 nism which effects the passage of the spemi head through the 

 surface of the egg is the same one which effects the continued 

 movement of the spemi head once it has entered the egg. 



Lillie (1912b) considered the fertilization cone of Nereis to be 

 quite different from the fertilization cone of echinoderms since 

 in the latter "its significance has been, apparently, merely tem- 

 porary and local, a reaction of the ovum to the spermatozoon 

 with no definable function of its own." In the light of present 

 knowledge this view certainly merits reconsideration. There is no 

 doubt that the formation of an outwardly projecting fertilization 

 cone is a normal phenomenon at least in the eggs of some species. 

 In these the cone is not simply a result of pressure or other con- 

 ditions introduced by placing eggs on a slide and covering them 

 with a coverslip. In Saccoglossus kowalevskii, for example, the 

 relatively large fertilization cone may be seen with the dissecting 

 microscope in eggs inseminated and left in ample water in open 

 Syracuse or stendor dishes. Such eggs develop normally to larval 

 stages (Colwin and Colwin, 1953). Moreover, in at least three 

 echinoderms, T. hriareus, H. atra, and A. forbesii, the acrosome 

 filament which initially may be seen within the outwardly project- 

 ing part of the cone subsequently moves into the egg proper. 

 Further study of these species may reveal a mechanism similar to 

 the one described for Nereis. The great difference in time re- 

 quired for spemi penetration in Nereis as compared with other 

 species may not reflect any fundamental difference in mecha- 

 nism. 



Summary 



1. From the evidence reviewed it is clear that in certain echino- 

 derms, molluscs, annelids, and probably an enteropneust, sper- 

 matozoa undergo a profound reaction when suitably stimulated. 

 The most striking feature of the reaction is the production of a 

 fairly straight, relatively rigid filament, the acrosome filament of 

 Dan. Egg water, alkaline sea water, and the presence of unferti- 



