424 HISTOLOGY 



nourishing cell. The lumen of a lobule will be considered as distal in 

 direction and the capsule as proximal. 



The male gonad differs histologically according to the season and 

 the ways in which the sperm is matured. Some organisms mature but 

 one lot of sperm in a lifetime, and others mature it from a very different 

 and newly developed testis each year. In such a form we are apt to 

 find a lack of the sperm column segregation and to find that each lobule 

 matures all its sperm at once and in a mass. Such a lobule is not a per- 

 manent structure, but is destroyed immediately after the discharge of 

 the spermatozoa. 



In the other animals the sperm may be produced for long periods or 

 even continuously, as in man. Here the lobule is usually a permanent 

 structure and a residuum of living reproductive cells, as spermatogonia, 

 is always to be found on the basement membrane. At certain periods, 

 determined in man by "waves" of successive maturation periods which 

 pass down the tubules, some of these spermatogonia begin to undergo 

 maturation. As they begin to mature and develop they leave their 

 basal position and move distally in successive layers, meanwhile going 

 through the maturation stages, until when they arrive at the lumen 

 they become functional male reproductive cells. 



The nurse cells commonly remain on the basement membrane. 

 This obliges the growing spermatids to move to them and remain in a 

 proximal position until discharged. All the cells of a single sperm col- 

 umn commonly mature together. In the skate we find a long, sea- 

 sonal, sperm-production period during which a series of new spermatic 

 lobules are being continually formed from a germinative center. As 

 these mature they move away from this center until, when ripe, they 

 are ruptured and destroyed at the surface of the testis, setting the sper- 

 matozoa free into the seminal ducts. These lobules show a well- 

 defined sperm column arrangement. 



The development of a spermatogonium into four ripe spermatozoa is 

 one of the most interesting of known cytological processes. The stages, 

 divisions, etc., through which they go are exactly homologous to those to 

 be later described for the female reproductive cells. We shall describe 

 these stages simply and shortly at first. 



Beginning on the basal layer as a spermatogonium, the cells await the 

 breeding season, and having gone through the contraction stage or synizesis 

 each one grows in size to become a spermatocyte of the first order. This 

 spermatocyte now lies, as a rule, in the second layer of the reproductive 

 epithelium and rapidly goes through with its first reduction division, 

 which results in the production of two spermatocytes of the second order. 

 The chromatin is arranged and divided as will be described in detail 

 below and, sometimes without re-forming their nuclei, the second sper- 



