230 MICROSCOPIC ANATOMY OF THE ORGANS. 



selves, not touching the membrana propria. Such modified 

 spermatogonia containing much protoplasm are known as sper- 

 matocytes of the first order. Mitotic changes occur in the 

 nuclei of these, and a double division takes place. By the first 

 division there are formed the spermatocytes of the second order, 

 and by the second division the spermatids arise. According to 

 many authors, there is a short resting stage between these two 

 divisions. The first is spoken of (Flemming) as hetero typical, 

 and the second as homotypical. The difference between the two 

 is in the fact that in the diaster stage of the first there is a 

 repeated longitudinal splitting of the chromosomes, while in 

 the homotypical division this does not occur. It is in this 

 period of division that the reduction of chromosomes takes 

 place (see Fertilization). 



The spermatids are small cells bordering on the lumen of 

 the tubule. They represent the last generation of the sexual cell, 

 and are converted into the spermatozoa. During this process 

 of changing, the cells of Sertoli become modified. Their proc- 

 esses extend into the lumen, and the spermatids, which are to 

 be converted into the spermatozoa, group themselves around 

 each process to form what are known as the spermatob lasts 

 (v. Ebner). In this way they become nourished, and after a 

 certain time the combination becomes looser, and finally the 

 newly formed spermatozoa move away. 



Numerous investigations have been carried on in late years 

 to arrive at the exact method of transformation of the sperma- 

 tids into the spermatozoa, but the entire process is still by no 

 means clear. Since it is not possible here to discuss the various 

 views that are held with regard to this, we shall confine our 

 description to the process as it occurs in man and the other 

 mammals, and follow in the main the investigations of Meves. 

 In all mammals the process is very similar. Observing the 

 parts of Fig. 178, which represents the transformation of 

 human spermatids into spermatozoa, we notice that the chro- 

 matin matter of the spermatid nucleus shows a progressive 

 increase in density and a more and more homogeneous appear- 

 ance. The nucleus is originally round and centrally placed, but 



