TESTIS 



335 



The sexual or genital cells are apparently produced from the cords in the testis, 

 relatively late in embryonic development. It was suggested by Nussbaum, however, 

 that the sexual cells are set apart much earlier "they do not come from any cells 

 that have given up their embryonic character or gone into building any part of the 

 body." In accordance with this idea, it is considered by some authorities that in the 

 segmentation stages, a line of undifferentiated cells is set apart to become the sexual 

 cells, add that from the beginning they are distinct from the somatic cells which form 

 the rest of the body. As stated by Allen (Journ. Morph., 1911, vol. 22, pp. 1-36), 

 the sexual cells do not belong to any one germ layer; they are free to follow their own 

 path in their travels from the place of origin to the genital glands where they finally 

 come to rest. Thus the sexual cells have been reported as distributed somewhat 

 diffusely in the entoderm and mesoderm. (For papers on this subject, see Allen, 

 Anat. Anz., 1906, vol. 29, pp. 217-236.) In a human embryo of 2.6 mm. Felix found 

 seven of these large 'clear cells, all in the immediate vicinity of the cloaca. Another 

 embryo of 2.5 mm., showed twelve "primary genital cells." But he adds, that they all 

 disappear in later stages, and when the genital glands are formed there are no genital 

 cells. At present it has 'by no means been demonstrated that the mammalian sexual 

 cells are not differentiated products of the testis or ovary, adapted for the special pur- 

 pose of reproduction. 



^g 



FIG. 333. CROSS SECTION OF A CONVOLUTED 

 TUBULE OF THE TESTIS AT BIRTH. 

 (Eberth.) 



FIG. 334. SUSTENTACULAR CELLS. 



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



(B&hm and von Davidoff.) 



The sustentacular or supporting cells, often called Sertoli's cells, are 

 at first indifferent cells forming a syncy tium (Fig. 333) . With the increase 

 in the number of spermatogonia, their protoplasm is resolved into a 

 network of strands, molded by the surrounding cells (Fig. 334). Their 

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

 protoplasm extending from the periphery of the tubule toward its lumen. 

 Each nucleus has a distinct nucleolus, apart from which its chromatic 

 material is very scanty. Usually the nuclei are in the lower half of the 

 branching protoplasmic columns, the polygonal bases of which are in 

 contact with one another beneath the spermatogonia. Within the proto- 

 plasm fat droplets occur, together with brown granules; crystalloid 

 bodies in pairs may also be found. As seen in Fig. 334, a, the heads of 

 the spermatozoa appear attached to, or imbedded in, the protoplasm 

 of the sustentacular cells, which are supposed to nourish them. The 



