270 HISTOLOGY. 



The process of their formation from spermatogonia is known asspermat- 

 ogenesis. In place of the name spermatozoon which was applied to 

 seminal filaments when they were considered parasitic organisms, the 

 term spermium has been proposed. 



S per mato gene sis. The spermatogonia which are found at the periph- 

 ery of the convoluted tubules, divide by ordinary mitosis for a variable 

 number of times. Some of the resulting cells move toward the lumen and 

 increase considerably in size. The chromatin in the nucleus of each 

 forms a thread which is resolved into one half the usual number of chro- 

 mosomes. Before this takes place the chromatin may be gathered at one 

 side of the nucleus, a condition named synapsis; ordinarily it appears 

 as a convoluted thread or spireme. The reduced number of chromo- 

 somes in man is said to be twelve, in favor of which the drawing copied 

 as Fig. 306 has recently been published. The chromosomes are seldom 

 so arranged as to allow a conclusive count. Instead of being of the usual 

 elongated form they are block-like bodies which in 

 certain animals are each clearly groups of four 

 granules or subdivisions. They become ring- 

 shaped before dividing in halves (the ring-shaped 

 arrangement characterizing the heterotypic form of 

 mitosis) and each half contains two of the four 

 granules. The cells produced by this division are 

 somewhat smaller and pass toward the lumen. 

 FlG ' io'<^E! MA H Y u S M A R N Within their nuclei the chromatin returns to the 



(l (Af? spireme form, and possibly to a network. The 

 chromatic thread again is resolved into twelve chro- 

 mosomes, each in some animals consisting of two granules. In the mitosis 

 which follows, these divide into single granules and each of the cells pro- 

 duced receives twelve. They then form a network in a small nucleus, the 

 entire cells being reduced in size. These cells border upon the lumen. 

 The generations of cells which have been described are named as follows. 

 Those which proceed from spermatogonia, and which first present the 

 reduced number of chromosomes are called primary spermatocytes. They 

 are large cells in the outer part of the tubule, sometimes with vacuolated 

 protoplasm containing rows of granules. Each of them divides into 

 two secondary spermatocytes (praespermatids) which are similar cells, 

 though smaller and nearer the lumen. They also have the reduced 

 number of chromosomes. Every secondary spermatocyte divides into 

 two decidedly smaller spermatids, giving them the reduced number 

 of chromosomes. The spermatids without further division are trans- 

 formed into spermatozoa. Thus each primary spermatocyte produces 

 four spermatozoa. 



