spermatogenesis of the Mongoose, etc. 169 



synapsis, and can thus produce no confusion at those stages where hetero- 

 chromosomes when present are most conspicuous, i. e., synaptic and early 

 postsynaptic stages. The conclusion that heterochromosomes are wanting 

 in the male of the mongoose, cat, squirrel, pig, and rabbit is, of course, based 

 on the assumption that if present they would be conspicuous at the same 

 stages in which they are so strikingly in evidence in the group of mammals 

 next to be described. The final test must, however, in these cases be an 

 actual count of chromosomes. 



HETEROCHROMOSOMES PRESENT IN MALE. 



WHITE MOUSE. 



Spermatogonial and spermatocyte resting nuclei agree in being vesicular 

 and having a delicate chromatic reticulum with numerous net-knots (figs. 

 22 and 23) ; they differ in that the spermatocyte nucleus has a bilobed or 

 double chromatin (chromosome) nucleolus. 



The Sertoli cell has a still more vesicular nucleus, which is very con- 

 spicuous by reason of a characteristic trilobed, deep-staining nucleolus, the 

 central section being invariably the larger. This peculiar type of nucleolus 

 is as characteristic of the Sertoli cell as its peculiar shape, and suggests 

 interesting speculations concerning the relationship between nucleolar 

 morphology and cell functions. 



My earlier observations led me to the conclusion that the synapsis 

 phenomenon {i. e., synaptic knot, contraction or bouquet phase, or synizesis 

 stage) 1 was lacking in the auxocytes of the white mouse. Further careful 

 search has disclosed a few cells in the condition illustrated in figure 24. 

 This is tentatively interpreted as synapsis (polarized amphitene). If the 

 nucleus is normal, as it appears to be, the paired threads unmistakably 

 indicate synapsis. I find that Regaud (1909, 1910) reports his failure to 

 find a synapsis phase also in the rat. This phase is assuredly not accentu- 

 ated in white mouse. Nothing more closely resembling synapsis than the 

 nuclear arrangement illustrated in figure 24 could be found in my material. 

 This, however, seems sufficiently suggestive of true synapsis to show that 

 something similar to synapsis actually occurs in the mouse, perhaps too 

 rapidly to appear except very occasionally in "fixed" preparations. If the 

 phenomenon is really lacking the "synapsis" figure does not appear essen- 

 tial to the pairing of the chromosomes in the formation of the haploid group. 

 During synapsis (parasynapsis) and early postsynapsis (figs. 24 and 25) 

 the chromatic accessory body (chromosome) is apparently usually single, 

 usually oval, sometimes slightly bilobed, and occasionally wedge-shaped or 

 even irregular. During later postsynaptic (fig. 26) and prophase (fig. 27) 

 stages, this element again appears invariably double, frequently in the form 

 of a longitudinally split rod. There is no unequivocal evidence that the 



> Synapsis is assumed to take place during the period of contraction or " synizesis "; hence the terms are 

 here used interchangeably. 



