1124 



THE REPRODUCTIVE ORGANS 



Nucleus.. 



End-knob. 



Middle-piece. 



Envelope of the tail. 



■ Axial filament. 



lium, and descend later on through the abdominal ring into the gradually enlarg- 

 ing scrotum. Their descent through the ring takes place shortly before birth. 

 This fact also accounts for the peculiar blood supply of these organs which is de- 

 rived from the abdominal aorta by the slender and unusually long spermatic 

 arteries. The venous return is effected by the spermatic veins, the right one 

 entering the inferior cava directly, and the left one, the left renal vein. Inasmuch 

 as the latter joins the renal almost at right angles, it cannot discharge its blood 



with absolute freedom, a condition which in later 

 years often gives rise to a venous engorgement 

 Apical body or acrosome. and a lower position of the corresponding organ. 



The Development and Character of the 

 Spermatozoa. — Up to the time of puberty, 

 the seminal tubules are filled with cells 

 containing unusually large nuclei. Among 

 these are found the spermatogonia which 

 then discontinue their divisions and rapidly 

 develop into the so-called spermatocytes. 

 From these arise by hetero-mitosis the sper- 

 matids or sperm-cells, and from these in turn 

 the adult spermatozoa. Each spermatocyte, 

 however, divides into two daughter-cells 

 and the latter in turn into two, so that 

 really four spermatids and spermatozoa are 

 developed from each primary spermatocyte. 

 The nuclear material of the spermatid is 

 transformed directly into that of the sper- 

 matozoon, while its cytoplasm is appor- 

 tioned to the tail. In some cases, however, 

 the centrosome of the spermatid is con- 

 verted into the middle piece and the axial 

 filament of the tail. This process, there- 

 fore, is not a mere division of the cell, but 

 a reduction-mitosis, the chromosomes being 

 reduced by one-half. In this regard, the 

 formation of the spermatozoon is analogous 

 to the maturation of the ovum during the 

 projection of the polar bodies, but since 

 the union of these two elements eventually 

 restores the original number of chromo- 

 somes, the spermatozoon and ovum really 

 supplement one another. 

 When fully formed, the spermatozoa are forced into the epididymis 

 and vas deferens, but since they do not become motile until they have 

 reached the former, their progress through the more distal channel 

 must be effected by the lining cells of the seminiferous tubules and 

 differences in pressure. In the vas deferens, they are then able to 

 unfold their power of movement more fully and, besides, this tube 

 greatly facilitates their progress by its peristaltic contractions as 



End-piece. 



Fig. 534.— Diagram of the 

 Flagellate Spermatozoon. 

 (From Wilson, "The Cell in De- 

 velopment and Inheritance.**) 



