2 68 



HISTOLOGY. 



fectly developed or absent. The sexual cells multiply slowly by ordi- 

 nary mitosis, until puberty when their increase in number becomes rapid. 

 In a somewhat smaller form with round nuclei containing abundant 

 chromatin, in granules or encrusted at the nuclear membrane, they are 

 called spermatogonia. From them the mature sexual cells are derived. 

 The cells which in Fig. 303 constitute the larger part of the tubule are 

 cajled sustentacular cells (Stertoli's cells, vegetative, or follicular cells). 

 They form a syncytium and with the increase in the number of spermat- 

 ogonia their protoplasm is resolved into a network of strands. Their 

 nuclei are radially compressed into ovoid shapes and lie in columns of 

 protoplasm extending from the periphery of the tubule toward its lumen 

 and moulded by the surrounding cells. Each nucleus has a distinct nucle- 

 olus apart from which its chromatic material is very scanty. Usually 

 the nuclei are in the lower half of the branching protoplasmic columns, 



FIG. 303. CROSS SECTION OP A CONVOLUTED 

 TUBULE OF THE TESTIS AT BIRTH. 

 (Eberth.) 



FIG. 304. SUSTENTACULAR CELLS. 



a., Isolated (Koelliker) ; b., Golgi preparations 



(Bohm and von Davidoff.) 



the polygonal bases of which are in contact with one another beneath 

 the spermatogonia. Within the protoplasm fat droplets occur, together 

 with brown granules; crystalloid bodies in pairs may also be found. The 

 appearance of the sustentacular cells in ordinary sections is shown in Figs. 

 305 and 309, in which it is evident that they may be recognized by their 

 characteristic nuclei. 



There are two views as to the origin of the sexual cells. According 

 to the first they arise from the mesoderm quite like the cells of other organs; 

 the second regards them as a race of undifferentiated cells set apart from 

 the outset of development. In the worm, Ascaris, it has been observed 

 that the fertilized ovum divides into two cells, one of which produces only 

 somatic cells (those of the various tissues) and the other divides into a 

 somatic and a sexual cell. In the mitoses which follow, the sexual cell at first 

 continues to gve rise to a somatic and a sexual cell, but later its products are 

 wholly sexual. In certain fishes large cells situated in the entoderm and meso- 

 derm before the genital glands have formed, are regarded as sexual cells (germ 



