SIGNIFICANCE OF SPERM-HEAD IN CEREBRATULUS. 



3d 



round sperm-nucleus, which does not differ much from that of 

 the spermatid. 



When informed by Professor E. B. Wilson 

 that the egg of Cerebratulus marginatus has no 

 membrane (Fig. 1, B), I thought that if the above 

 conclusion were true, this species must have a 

 spermatozoon with a blunt head. At Naples in 

 the spring of 1906, I found that this expecta- 

 tion was fulfilled. Instead of the slender pointed 

 head found in Cerebratuius lacteus (10.6//), the 

 Neapolitan form (C. marginatus) has a sperma- 

 tozoon with a blunt head (5.4/*) terminating in a 

 knob as shown in Fig. 2, B. The length of the 

 tail is nearly the same in both forms. 



The difference in size of the sperm-heads 

 might be interpreted as due to the number of 

 chromosomes contained in them. In fact C. 

 lacteus has 18 or 19 chromosomes in the reduced 

 number, while C. marginatus has 16 according 

 to Coe. 1 Yet the difference in shape of the 

 sperm-head between two such closely allied 

 forms as these is difficult to explain without tak- 

 ing into consideration a special adaptation for 

 boring thick membranes (cf. Pfliiger and Smith, 

 '83 2 ). This, I think, is an actual instance to 

 support the general belief that the diversity in the shape of 

 sperm-head has evolved in response to the mechanical needs for 

 penetrating the egg. 



Zoological Laboratory, 

 Columbia University, 



New York, June 8, 1907. 



^oe, W. R., '99, "The Maturation and Fertilization of the Egg of Cerebratu- 

 ius. 1 '' Zool. Jahrb. Abth. Anat. u. Ont., 12. 



2 Pfliiger, E., und Smith, W. H., '83, " Untersuchungen iiber Bastardierung der 

 anuren Batracier und die Principien der Zeugung." Pfluger ' s Archiv, 32. 



Fig. 2. Sper- 

 matozoon of Cere- 

 bratuius lacteus 

 (A), and of C. 

 marginatus (■&)• 

 XH33- 



