THE SPEKMATOZOON. 17 



are formed by two successive mitotic divisions, of which the first is heterotypical 

 and produces reduction of the chromosomes, and the second is homotypical. 



The two divisions differ from the corresponding divisions of the oocytes in three 

 important respects : (1) centrosomes are present ; (2) the four grand-daughter cells 

 produced are of equal size and presumably of equal value, so far as capability of 

 uniting with a mature ovum to form a zygote is concerned ; (3) each of the four 

 grand-daughter cells possesses two centrosomes. 



In the prophase of the first or heterotype division the nucleus and nucleolus 

 disappear in the ordinary way. The centrosome divides, and an achromatic spindle 

 appears, which has the daughter centrosomes at its poles and half the typical 

 number of chromosomes at its equator. The chromosomes are twin chromosomes. 

 During the metaphase the two segments of each twin chromosome separate from 

 each other. In the anaphase they travel to the opposite poles of the achromatic 

 spindle, and consequently, when the cell divides in the telophase, each daughter 

 cell or spermatocyte II contains a centrosome and half the typical number of 

 chromosomes. 



The second maturation division, which takes place without the intervention of 

 a resting stage, is of the homo type form. The centrosome divides, a new achromatic 

 spindle appears, and the daughter chromosomes gather at its equator. In the 

 metaphase the chromosomes divide into equal parts, which travel to the opposite 

 poles of the spindle during the anaphase, and when the telophase is completed the 

 grand-daughter cells, which are called spermatids, possess a centrosome and half the 

 typical number of chromosomes. In the resting stage which follows, the chromatic 

 particles become enclosed in a new-formed nucleus, and the centrosome, if it -has 

 not already divided, separates into two parts, one which lies nearer the nucleus and 

 is called the anterior centrosome, and another, farther from the nucleus, termed the 

 posterior centrosome (Fig. 22). Numerous mitochondria are present, and an in- 

 definite structure, called the accessory body, is also found in the cell protoplasm. 

 A spermatid, therefore, differs from a typical animal cell not only because it 

 possesses the chromatic substance of only half the typical number of chromosomes, 

 but also because it possesses an accessory body and two centrosomes. 



From Spermatid to Spermatozoon. The reader will have noted that the 

 female gametes become mature and ready for conjugation with male gametes 

 directly after the second maturation division is completed. In the case of the 

 male germ cells, however, the spermatids which result from the second maturation 

 division have still to undergo a complicated process of transformation before they 

 become converted to spermatozoa or mature male gametes. The process of 

 transformation takes place in association with the nurse cells in which the 

 developing spermatozoa become embedded. 



The details of the process of transformation are difficult to follow, and the 

 knowledge regarding them is still to some extent indefinite. Certain points, 

 however, are well established ; but before they are considered it is necessary that 

 the reader should be acquainted with the anatomy of an adult spermatozoon. 



THE SPEKMATOZOOK 



A spermatozoon is a minute organism consisting of a head, a neck, a body, 

 a tail, and an end-piece. Its total length is about 50 /x, that is, its length is 

 about the same as the diameter of the nucleus of the ovum. 



The head has the form of a laterally compressed ovoid. It is separable into 

 anterior and posterior portions, and the anterior portion is more or less completely 

 covered by a head-cap, which culminates in a sharp ridge. The length of the head 

 is about 4*5 /z. 



The neck is an extremely short constricted region which intervenes between 

 the head and the body. At its anterior end, where it joins the head, there is a 

 deeply staining anterior centrosome, and at its posterior end a similarly deep- 

 staining posterior centrosome, from which a deep-staining axial filament extends 

 posteriorly through the body and tail into the end-piece (Fig. 21). 



2 



