A NOTE ON THE ADAPTIVE SIGNIFICANCE OF THE 

 SPERM-HEAD IN CEREBRATULUS. 



N. YATSU. 



While studying the fertilization processes in the living eggs of 

 Cerebratulns lacteus at South Harpswell, Me., I was struck by the 

 fact that it took the spermatozoa considerable time and not a 

 little effort to bore through the thick membranes in order to reach 

 the egg. From this I concluded that the long, slender and 

 slightly curved head of the spermatozoon of Cerebratulns lacteus 

 might have evolved in correlation with the thick egg-membrane 

 characteristic of this species (Fig. i, A)} 



B 



Fig. I. Egg of Cerebrahdus lacteus, {A), and of C. marginatus (B). X 22 °- 



Since the study of sections was begun this conclusion has been 

 strengthened. The spermatid has a round head. Later the an- 

 terior portion of the head is gradually drawn out into a slender 

 beak, the head proper still remaining pear-shaped. At the last 

 stage of this transformation the beak thickens and the head proper 

 elongates. Thus the typical shape of the sperm-head is attained 

 (Fig. 2, A). After entering the egg, the sperm-head repeats in 

 reversed order the process just described, finally giving rise to a 



1 The membrane is made up of two layers. By accident spermatozoa sometimes 

 find their way into the space between them. 



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