120 



FUNDAMENTALS OF CYTOLOGY 



the poles of the mitotic figure, so that cytokinesis separates them into 

 more or less equal groups (Fig. 28). Only in a few known cases does this 

 involve an actual division of individual chondriosomal bodies. In some 

 spermatocytes the Golgi bodies form a conspicuous mass, the idiosome, 

 about the centrioles, and this breaks up into smaller portions during the 

 divisions. The centrioles that function in the second meiotic division 

 have usually become doubled by the time the division is completed. As a 

 result of these events each spermatid consists of cytoplasm, a nucleus with 

 the gametic chromosome number, chondriosomes, one or more Golgi 

 bodies, a pair of centrioles, and frequently other inclusions. 



Fig. 89. — Stages in the transformation of the animal spermatid into a spermatozoon. 

 1, spermatid of an insect (Brochyviena). {Redrawn from R. H. Bowen.) 2-5, spermatids 

 of the guinea pig; ga, Golgi apparatus; gr, Golgi remnant; a, acrosome; n, nucleus; m, 

 mitochondria (in Brochymena they form a nebenkern); c', c^, centrioles; png, postnucloar 

 granules; pnc, postnuclear cap; mp, middle piece. {From J. B. Gatenby and H. W. Beams, 

 based on studies by Gatenby, Vejdovsky, and Meves.) 



The transformation of the spermatid into a spermatozoon, or sperm, is 

 known as sperniiogenesis. This process involves a very remarkable series 

 of changes which have been found to occur, with minor variations, in 

 animals of many types. Each of the spermatid components behaves in a 

 characteristic manner (Fig. 89). From one of the two centrioles a slender 

 filament grows out and pierces the cell membrane; this is the axial fila- 

 ment of the future tail of the spermatozoon. As the filament continues to 

 grow, the centrioles become modified and connected Anth the nucleus in 

 various ways. 



The chondriosomes in mammalian spermatids become grouped in a 

 region near the posterior pole of the nucleus, where they form a compact 

 sheath about the proximal portion of the axial filament. In insect 

 spermatids the chondriosomes usually unite into a single large mass, the 

 nebenkern, which soon divides and elongates as two long strands wound 

 about the axial filament in the lengthening spermatid. 



