CH. I] FORMATION OF THE SEX-CELLS 5 



seen in Fig. 7, A, B, C. It is probable that a very thin layer 

 of cytoplasm covers the outer surface of the head. The chro- 

 matin is densely packed into the head-piece, and cannot be 

 resolved into its component chromosomes. The middle piece 

 lies just back of the head. 1 In some animals this middle piece 

 is known to enter the egg with the spermatozoon and a part of 

 it becomes the centrosome, which then divides into two centro- 

 somes and around these arise the achromatic rays of the dividing 

 egg. The tail of the spermatozoon is generally described as 

 coming from the cytoplasm of the cell. 



The development of the spermatozoon in the salamander 

 has been carefully studied by Flemming ('87), vom Rath 

 ('93), Meves ('96), and others. There are certain remark- 

 able processes that take place in the spermatogenesis of 

 these Amphibia that seem to occur also in the frog, but as 

 they have not been as carefully worked out in the latter form 

 we may examine first the changes that take place in the sala- 

 mander. Each year after the male has lost its supply of 

 sperm, new spermatozoa begin to develop. The epithelial 

 cells lining the cavities of the testes divide at first after a 

 type of cleavage called, by Flemming, homoeotypic. This first 

 period of activity produces the first generation of spermato- 

 cytes, which divide according to another type, the heterotypic. 



The cells of the second generation of spermatocytes also di- 

 vide in the same way, but with an occasional homceotypic cleav- 

 age. Finally, in the third generation of spermatocytes, both 

 types of cleavage occur. The products of the third generation 

 transform directly into spermatozoa. In the heterotypic division 

 the process is as follows. The chromatin is at first arranged in 

 a thick thread, having a definite arrangement. The skein-stage 

 follows, and a longitudinal splitting of the chromatin-thread is 

 apparent. A thickening of the thread then takes place, and 

 it breaks up into twelve chromosomes (only half the number 

 present in other cells of the body) (Fig. 3, A, B). At the free 

 ends of the bent chromosomes, each of which is split longitudi- 

 nally, the halves fuse together (Fig. 3, B), but elsewhere the 



1 Its origin in the frog has not been definitely made out. It is probably 

 cytoplasmic in origin (Fig. 7, A, Bj C). 



