8 FERTILIZATION 



the egg pull the spermatozoon towards the egg surface, have been 

 made elsewhere; for example, Colwin & Colwin (1949) reported 

 that a thread-like structure connected the fertilizing spermatozoon 

 to the fertilization cone in the egg of Saccoglossiis kowalezoskyi (A. 

 Agassiz), while Monroy (1948) refers to the fertilizing spermato- 

 zoon of Pomatoceros triqueter (Linn.) being connected to the egg 

 surface by a thread. In the case of the starfish spermatozoon, 

 J. C. Dan (1954) has shown that in certain circumstances, a thin 

 filament, about 25 yu, long and 0-13 /x in diameter, can be observed 

 protruding from the front end of the head. Although immature 

 eggs respond to insemination, and therefore to sperm-egg collisions, 

 by emitting filament-like structures (E. B. Harvey, 1938), Dan's 

 work leaves little doubt that Fol, R. Chambers and Horstadius 

 were wrong in thinking that the starfish egg responds to a nearby 

 spermatozoon by emitting a filament which joins the egg to the 

 spermatozoon and pulls the latter towards the egg surface. The 

 presence of long filaments on the front ends of sperm heads may 

 be of wider incidence than has hitherto been realised. Rothschild 

 & Tyler (1955), for example, have reported their occurrence in the 

 spermatozoa of Echinocardiimi cor datum (Pennant), Mytilus edulis 

 (Linn.), Strongylocentrotus purpuratus (Stimpson) and Lepido- 

 chitona cinerea (Linn.). There are, however, some spermatozoa, 

 e.g. those of the bull and ram, in which such filaments do not exist. 

 The subject of acrosomal filaments and their role in fertilization is 

 still very much in its infancy. In a recent paper, J. C. Dan (1955) 

 has adduced convincing evidence that the spermatozoa of Japanese 

 sea-urchins eject acrosomal filaments in the presence of sea water 

 in which eggs of the same species have been standing, though the 

 reaction does not occur if the calcium content of the medium is 

 reduced. Do some spermatozoa always have acrosomal filaments 

 on their heads and others only after responding to some stimulus ? 

 Further experiments are needed to resolve this interesting and 

 important question, which has been brought into prominence 

 mainly through the work of J. C. Dan. 



Fertilization membrane. Unfertilized echinoderm eggs are sur- 

 rounded by a vitelline membrane outside the plasma membrane. 

 Fig. 3, At fertilization and shortly after the cortical change, the 

 vitelline membrane separates from the egg surface, the separation 

 starting at the point of sperm attachment and passing progressively 

 over the egg surface (Kacscr, 1955). After this, the vitelline mem- 



