208 



THE ESSENTIALS OF HISTOLOGY. 



two different phases of development may be observed, and in different 

 tubules of the same testicle every phase may be traced. The accompanying 

 diagram (fig. 241), which is constructed from drawings by H. H. Brown, 

 illustrates the cycle of changes above described : it is divided into eight 

 parts, each of which shows the condition of the epithelium of a seminiferous 

 tubule at a particular stage. 



Each spermatoblast becomes converted into a spermatozoon in the follow- 

 ing manner (fig. 242). The nucleus forms the head, while the tail develops 

 as a fine filament within, but subsequently growing out from, the protoplasm, 

 and apparently connected with the nucleus almost from the first. The 

 protoplasm appears to assist in forming the middle piece of the spermatozoon ; 

 but a considerable portion of the protoplasm of each daughter-cell con- 

 taming a number of small darkly staining particles (seminal granules) always 

 becomes detached and disintegrated before the spermatozoon is fully mature 

 (fig. 241, ,, s'). 



i 



FIG. 242. SPEKMATOBLASTS FROM THE RAT IN DIFFERENT STAGES OF DEVELOPMENT. 



(H. H. Brown.) 



1-6, developing spermatozoa from the testicle ; 7, a mature spermatozoon from the vas deferens. 

 The remains of the protoplasm of the cell, which is seen in 6 still adhering to the middle 

 piece of the spermatozoon and containing a number of dark granules, is thrown off as the 

 spermatozoon matures. 



