SPERMATOZOA. 77 



In Ilj/la arbor ea the head is spindle-shaped, acutely 

 pointed in front, obtusely behind ; the tail filiform, 

 and about twice as long. 



In Bana esculenta and arvalis the head is staff- 

 shaped and obtuse in front, and about half as long as 

 the filiform tail; aud in Bana temporaria and agilis 

 the head is very long and thin, not or but scarcely 

 longer than the tail, and attenuate into a fine point at 

 both ends. 



The spermatozoa of B. grseca and latastii bear a 

 greater resemblance to those of B. arvalis, but have a 

 more elongate head, the diameter ten to twelve times 

 in the length ; the head is cylindrical, feebly attenuate, 

 and quite obtuse at both ends. 



The spermatozoa of B. earner ani and B. iberica are 

 still unknown. 



Attention has been drawn above to the importance 

 of securing for comparison the seminal elements in 

 their fully ripe condition. La Valette St. George has 

 shown that at certain stages of development these are 

 almost identical in B. esculenta and B. temporaria ; 

 and that accumulations of protoplasm at certain 

 points of the unripe element or spermatocyte, as he 

 calls it, produce a shape which may be very unlike 

 that of the true spermatozoon. So careful an observer 

 as Leydig has unfortunately fallen into the error of 

 figuring spermatocytes as spermatozoa in Bombinator, 

 Bufo, and Hyla. 



Pfltiger has observed that spermatozoa w T ith pointed 

 heads possess a greater facility for penetrating the 

 envelops of the ova of other species to reach the 

 nucleus, a fact which explains the different results 

 obtained in attempts at crossing in both directions, 

 to which highly interesting experiments allusion will 

 again be made in the chapter on hybrids. 



