350 THE ORGANS 



Enveloping the nuclear material of the head is a thin layer or delicate 

 membrane of cytoplasm, the galea capitis. The front of the head 

 ends in a sharp edge, the apical body or acrosome. In some lower 

 forms the acrosome is much more highly developed than in man and 

 extends forward as a pointed or barbed spear, the perforatorium. 

 The acrosome is differentiated from the nuclear portion of the head 

 by taking an acid dye, the chromatin, of course, taking a basic stain. 



The body is cylindrical, about the same length as the head, and 

 consists of a iibrillated central core, the axial thread, surrounded by 

 a protoplasmic capsule. A short clear portion, the neck, unites the 

 head and body. Just behind the head the axial thread presents a 

 bulbous thickening, the anterior end knob, which fits into a depression 

 in the head. At the junction of neck and body are one or several 

 posterior end knobs to which the axial thread is attached. The 

 latter leaves the body through a perforated ring, the end ring or end 

 disc. Delicate fibrils — spiral fibres — run spirally around the body 

 portion of the axial thread. 



The tail consists of a main segment, from 40 to 6o/« in length, and 

 a terminal segment having a length of from 5 to 10/'.. The main seg- 

 ment has a central fibrillated axial thread which is continuous with 

 the axial thread of the body. This is enclosed in a thin cytoplasmic 

 membrane or capsule continuous with the capsule of the body. This 

 membrane, inconspicuous and apparently structureless in man, is 

 remarkably developed in some lower forms, e.g., the membrana 

 undulata of birds. The terminal segment consists of the axial thread 

 alone. The motility of the spermatozoon depends entirely upon the 

 flagellate movements of the tail. In many of the lower animals the 

 spermatozoon has a much more complicated structure. 



Of the above-described parts of the spermatozoon only the head and 

 tail can usually be differentiated, except by the use of special methods and 

 very high-power objectives. 



Development or the Spermatozoa. — As already noted in describing the 

 testicle, the spermatozoa are developed from the epithelial cells of the semi- 

 niferous tubules. The most peripheral of the tubule cells, the spermatogones 

 (Fig. 235, sp and Fig. 236, sp) are small round cells with nuclei rich in chromatin. 

 By mitosis the spermatogone gives rise to two daughter cells, one of which 

 remains at the periphery as a spermatogone, while the other takes up a more 

 central position as a spermatocyte (Fig. 236, sc and Fig. 238, sc). The latter are 

 rather large spherical cells, whose nuclei show verj' distinct chromatin networks. 

 By mitotic division of the spermatocytes of the innermost row are formed the 

 spermatids (Fig. 236, st and Fig. 238, si). These are small spherical cells, which 



