THE TESTIS 379 



small spheroidal cells with little cytoplasm and large spherical 

 nuclei. Unlike the nucleus of the spermatocytes, that of the 

 spermatid has a well defined nuclear membrane and its chromatin 

 is distributed in irregular karyosomes. Mitosis is at an end ; and 

 each spermatid continues its development by direct transformation 

 into a spermatozoon. At first the spermatids form an innermost 

 group of independent cells ; later they arrange themselves about 

 the extremities of the Sertoli cells to which they become firmly 

 united. In this way they enter into the formation of the sper- 

 matoblast of von Ebner. 



The Sertoli cells are of large size and irregular columnar form. 

 They possess an ovoid vesicular nucleus with a distinct nuclear 

 wall and prominent nucleolus, but are otherwise poor in chromatin, 

 a fact by which they can usually be distinguished from the neigh- 

 boring cells, all of which are relatively rich in nuclear chromatin. 

 The cytoplasm of the Sertoli cell is finely granular, toward the 

 inner extremity often somewhat fibrillar ; and the base of the cell 

 frequently contains minute fat droplets. When first formed from 

 the primordial cells, which also give rise to the spermatogonia, 

 the Sertoli cells are cuboidal in shape and are relatively low; 

 moreover, the long axis of their ovoid nucleus is nearly parallel to 

 the basement membrane. As development proceeds and they 

 unite with the spermatids to form the spermatoblasts, the Sertoli 

 cells become greatly elongated, and their nucleus revolves until its 

 long axis is nearly at right angles to its former position ; it also 

 becomes more centrally placed. The surface of their cytoplasm 

 is indented by the attached spermatids and often presents short 

 lateral processes by which the cell is placed in relation with a 

 large number of the spermatid cells. As the spermatids develop 

 into spermatozoa the heads of the latter cells become deeply em- 

 bedded in the cytoplasm of the Sertoli cell, which is thus enabled 

 to contribute to the cytoplasm of the spermatosomes. The tail 

 pieces of the latter project centralward into the lumen of the 

 tubule. Finally when the transformation of the group of sperm- 

 atids into a corresponding number of spermatozoa is complete, 

 the newly formed germ cells break away from the Sertoli cell and 

 become free in the lumen of the seminiferous tubule. 



The spermatids become spermatozoa by a process of direct 

 transformation, the nucleus of the former producing the nucleus, 

 and its cytoplasm the middle and tail pieces of the latter. In 

 this process the nuclear chromatin becomes much condensed and 



