A. L. COLWIN AND L. H. COLWIN 155 



continue to enlarge even after the sperm head and its acrosome 

 filament are well within the egg proper. The cones are always 

 broad. (2) The acrosome filament enters the egg intact as the 

 most anterior part of the sperm head and remains essentially un- 

 altered in diameter and length until at least well into the egg. 

 With less favorable material one might erroneously have con- 

 cluded that the filament connected the sperm head with the 

 apex of the cone and that the filament contracted during sperm 

 entry. (3) At no time was there any basis for confusion between 

 the straight rodlike acrosome filament and the numerous coarse 

 radial striations which occur in the jelly hull (cf. Fig. 37 with 

 Figs. 38-39). 



Thyone briareus. The process of sperm entry and cone forma- 

 tion in this holothurian is similar to that of H. atra in many re- 

 spects, but it differs in certain significant details (Colwin and 

 Colwin, 1956). The observations were made on eggs removed 

 from the gonads and also on naturally shed noniial eggs. Eggs 

 from both sources were polyspermic in coverslip preparations. 

 Control cultures of artificially obtained eggs did not cleave, but 

 the control cultures of the naturally shed eggs gave 98-99% 

 cleavage and more than 90% active larvae. The chief differences 

 between the entry phenomena in eggs from the two sources were 

 that in the artificially obtained eggs sperm entry was slower and 

 the cone rose higher around the acrosome filament than in the 

 naturally shed eggs. 



The egg is surrounded by a jelly hull of about 55 microns in 

 thickness. Through this jelly run radial striations which are 



Figs. 37-45. Unretouched photographs of living preparations showing 

 sperm entry in Holothuria atra. (From Colwin and Colwin, /. Morphol. 

 1955a.) 



Fig. 37. Portion of uninseminated egg showing jelly hull with filamen- 

 tous radial projections from the egg surface. 



Fig. 38. A reacted spermatozoon with the acrosome filament in contact 

 with the egg surface (no cone formed); spermatozoon did not enter egg. 



Figs. 39—44. Successive stages of sperm entry into the egg (taken from 

 several eggs); note the entry of the intact acrosome filament into the egg 

 proper as the most anterior part of the sperm head. 



Fig. 45. Compressed cone with exudate emerging from ruptured re- 

 gion; spermatozoon in cone appeared unaffected. 



