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AMERICAN JOURNAL OF VETERINARY MEDICINE 



ber of these bodies is now evidently reduced 

 one-half, excluding the mysterious sex-chro- 

 mosome. This reduction in number is very 

 significant, since it is necessary in order to 

 keep the number constant for each species of 

 animal. It will readily be seen now that 

 when a father cell and a mother cell, each 

 with half the normal number of chromosomes, 

 unite, that the daughter cell will possess the 

 normal number of the parents — the daughter 

 the number of the mother and the son that of 

 the father. Should this fortunate process not 

 happen, and the mother and father pass to 

 the child their normal number instead of one- 

 half, our cells would soon be bursting with 

 chromosomes and thus future production 

 would be impossible. 



The compact mass of chromatic fibers now 

 break up and are again faintly distributed 

 through the nucleus (Fig. 5). As the cell 

 still continues to grow, this faint chromatin 

 material collects into as many aggregations 

 as there were pairs of fibers (Fig. 6). These 

 masses now condense and form the chro- 

 mosomes for the first of the two ripening 

 divisions (Figs. 7, 8 and 9.) 



B. The two ripening divisions. The germ 

 cells now undergo two divisions, differing 

 somewhat from those discussed earlier. The 

 number of chromosomes in these divisions is 

 one half, plus, the sex-chromosomes which 

 had no mate in the growth period. This fe- 

 male producing body still (in most forms and 

 in all domestic animals) refuses to associate 

 with the other chromosomes but takes its po- 

 sition at one side of the spindle, but in ad- 



vance of its neighbors (Figs. 7 and 8). All 

 the common chromosomes divide as usual and 

 the respective halves pass into the daughter 

 cells. Since the sex-chromosome passes un- 

 divided to one cell we have two different types 

 of cells produced. It will be noted later that 

 the one with the extra body (Figs. 8, 10 and 

 11) will produce a female while the one with- 

 out this important chromosome (Figs. 9, etc.) 

 will give a male. In some forms (not in 

 domestic animals) the sex-chromosome di- 

 vides in the first but passes bodily to the 

 daughter cell in the second division, the re- 

 sulting process being the same. 



These two types of cells now divide once 

 more, all chromosomes taking part in the 

 process, and then pass into another state of 

 reconstruction. These reconstructing cells are 

 now called "spermatids" (Fig. 10) and are 

 transformed directly into sperms (Fig. 11). 

 This process of transformation is very com- 

 plicated and has been worked out to the most 

 minute details. The most important point to 

 consider here is the fact that all the chromo- 

 somes, except the female-producers, again pass 

 into the fibrillar condition. In some forms 

 the sex-chromosomes can be followed step by 

 step into the head of the mature spermato- 

 zoon. 



VII. A Typical Ripe M.\le Germ Cell. 

 A mature spermatozoon (Fig. 11) consists of 

 a head, neck or middle piece, and a tail. At 

 the forward point of the head there is a 

 small sharp body, the acrosome, by means of 

 which the sperm bores its way into the egg 

 (ovum Fig. 12). The chromatin material is 



Explanation of Plate 



The small active cells at the top of the 

 plate are known as spermatogonia! (in the 

 male) and oogonial (in the female) cells. 

 They represent a few of the many divisions 

 which must take place before a single germ 

 cell of a tiny embryo can become the large 

 reproducing testis or ovary of the adult ani- 

 mal. 



Figures 2 to 6 inclusive and the correspond- 

 ing figures to the right show some of the 

 more important steps in the growth periods 

 of the ripening germ cells. 



At the stages shown in Figures 4 and 5, the 

 germ cells of the mule and other hybrids 

 first begin to show indications of abnormality 

 which results in the final destruction of the 

 cell. 



Fig. 7. First ripening division of the male 

 germ cell. (First spermatocyte division.) 



Fig. 19. Corresponding division in the 

 ovary. (First oocyte division.) 



Figs. 8 and 9. Second ripening division. 

 (Second spermatocyte division.) 



Fig. 18. Second oocyte. 



Fig. 10. Spermatids (first two with and 

 the second two without the sex-chromosome). 



Fig. 11. Mature spermatozoa. 



Fig. 16. The single large mature egg. 



Fig. 17. The three small eggs (the polar 

 bodies) which never mature. 



Figs. 12, 13, 14, 15, represent the possibility 

 of the single egg (Fig. 16) being fertilized by 

 any one of the four "brother" spermatozoa 

 shown in Fig. 11. Since the first two contain 

 the sex-chromosome, a union of either of 

 these with the egg would result in female off- 

 spring (Figs. 12 and 13), while the other two 

 would produce males (Figs. 14 and 15). 



Figs. 12, 13, 14 and 15 also show that "the 

 reduced number of chromosomes brought in 

 by each parent cell (Figs. 12 and 13) unite 

 to form the normal number for the offspring 

 (Fig. 15)," and that "the chromosomes from 

 each parent (Fig. 13) are equally distributed 

 in every offspring (Fig. 15)." 



Fig. 1. A typical animal cell. 



$ =Male. 



9 =Female. 



