124 The Spermatogenesis of Desmognathus Fusca 



mitosis, three features are to be noted; the first of these, is the rapidity 

 with which the divisions follow each other, witliout an intervening 

 interval of rest and growth. The effect of this is, theoretically, a re- 

 duction in the size of the nucleus of the grand-daughter cells one-half. 

 In ordinary mitoses, the nucleus, n, increases by growth to 2n, divides 

 so that the daughter-nuclei represent n: by growth each of these in- 

 creases to 3n, to be reduced in the ensuing division to n, and so on. 

 Omit one of the periods of growth so that the second division follows 

 immediately after the first_, and the nuclei in the daughter-cells of the 

 second division are reduced to -Jn. A quantitative reduction of the 

 nuclear matter to one-half is accomplished as an inevitable result of the 

 two rapidly following divisions. The difference in relative size of the 

 cells of the generations of the spermatocyte divisions may be easily seen 

 by comparing the figures, after reducing those of Plates III and 

 IV, as directed. The spermatogonium has a nucleus Avith a diam- 

 eter of, say, 25n, the nucleus of the mature spermatocyte measures 

 32n, that of the secondary spermatocyte has a diameter of 25n, while 

 the spermatid has, as the corresponding measurement, 18n. This, of 

 course, gives but the grossest idea of the size differences of the cells 

 and nuclei. The size of the nucleus depends in part upon the growth 

 period it has enjoyed, and this, in turn, must depend upon numerous 

 factors, among them the metabolic interrelation of nucleus and cell- 

 body, so that, considered quantitatively, the size of the nucleus is largely 

 relative and variable. In the divisions of the spermatogonia there is 

 such a variation in the size of the nuclei that it would be very difficult 

 to estimate the size relative to the original embryonic nuclei. The 

 primary spermatogonia possess large nuclei; these undergo rapid division 

 and there is a decrease in the size accordingly. The period of growth 

 of the spermatocyte again increases the size of the nucleus, restoring 

 it — may we assume? — to the original size before a division. Only in 

 case we assume that the quantity of nuclear matter in embryonic cells 

 remains approximately constant, and that the mature spermatocyte has 

 a nucleus as large as that of an embryonic cell before a division, is it 

 safe to state that the divisions of the spermatocyte accomplish a quanti- 

 tative reduction to (approximately) one-half. 



The second point that seems well established is that in the sperma- 

 tocyte mitoses the chromosomes appear in one-half the number that 

 has been found in the ordinary tissue (and embryonic) mitoses in the 

 respective forms. The significance that this seems to possess is the 

 prevention of the doubling of the chromosomes and the maintenance of 

 their numerical constancy in the species. It is, therefore, prophetic, 



