26 HISTOLOGY 



plate and in K it is passing to the upper pole of the spindle. In the sper- 

 matogonial divisions the accessory chromosome always divides with the 

 others; but in the division of the primary spermatocyte it passes un- 

 divided into one of the daughter cells. Thus one secondary spermatocyte 

 will receive eleven chromosomes (dyads) and the other will receive twelve 

 (eleven dyads and the accessory chromosome) . In the late anaphase shown 

 in Fig. 20, L, the accessory chromosome cannot be recognized. 



Secondary spermatocytes. The secondary spermatocytes pass rapidly 

 from the condition shown in Fig. 20, L, to that of Fig. 21, A. A nuclear 

 membrane has developed, and the dyads have become somewhat filamen- 

 tous. Without passing through a complete resting stage they proceed to 

 divide as shown in Fig. 21, B-F. The dyads separate into their com- 

 ponent halves. In those secondary spermatocytes which received the 

 accessory chromosome, that body will be seen dividing with the dyads, and 

 each spermatid will receive one-half of it. It has been questioned whether 

 the division of the accessory chromosome is longitudinal and there- 

 fore equational, or transverse and reductional. Many cytologists 

 consider that if a chromosome splits lengthwise, all of its parts will be 

 represented in the resulting halves, but if it divides transversely, essential 

 elements will be lost. This conception lends importance to the question 

 of transverse or longitudinal division of the accessory chromosome. By 

 the division of this chromosome it comes about that one-half of the sper- 

 matids contain twelve chromosomes, and one-half contain eleven, as indi- 

 cated in the diagram, Fig. 18. The spermatids shown in Fig. 21, F, con- 

 tain tEe accessory chromosome. 



Spermatids qnd Spermatozoa. In forming spermatozoa, the spermatids 

 become elongated, passing from the condition shown in Fig. 21, F, to that 

 of Fig. 21, G. The chromatin within the nucleus is distributed in fine 

 granules throughout the linin reticulum. Close to the nuclear mem- 

 brane a small dark body has appeared, from which a slender filament has 

 grown out. This body is usually described as the centrosome. A conden- 

 sation within the cytoplasm, seen also in F, is known as the paranucleus. 

 It is of uncertain origin, but may proceed from the cytoplasmic structure 

 called mitochondrium. The paranucleus forms a sheath about the axial 

 filament. 



Successively later stages are shown in Fig. 21, H, I, and J. The chro- 

 matin within the nucleus becomes homogeneous and very dense; at the 

 same time the nucleus elongates and forms the head of the spermatozoon. 

 This is enveloped by the cell membrane, but there is no appreciable layer 

 of protoplasm around it. The centrosome elongates and forms the middle 

 piece of the spermatozoon; and the axial filament, with a covering derived 

 from the paranucleus and cytoplasm, constitutes the tail. Only a portion 

 of the tail is included in the figure. The human spermatozoon likewise 



