166 PROCEEDINGS OF THE ACADEMY OF [Feb., 



early spermatogonic prophases, we can conclude that they must be 

 present in the rest stage of the nucleus, though merely in the form of 

 constituents of the chromatin reticulum. And their juxtaposition in 

 the chromatin spirem is a point in evidence of an earher contention of 

 mine (1900, 1904a), that in the chromatin spirem of spermatogonia 

 homologous chromosomes, i.e., such as unite into pairs during the con- 

 sequent synapsis stage, lie next each other. All the chromosomes 

 become longitudinally split during the prophases. 



Two clear pole views were found of the spermatogonic monaster 

 stage (metaphase) , Plate IX, figs. 7 and 8. Each showed exactly twenty 

 chromosomes. These occur in pairs, and we can distinguish three largest 

 pairs {A, a; B, b; C, c) and three smallest (in succession from the 

 largest to the smallest, F, f; D. d; E, e). The exact similarity in form 

 and size of the members of a pair does not evince itself so clearly in a 

 camera drawing as in the study of the chromosomes themselves, be- 

 cause the members of a pair usually do not lie exactly in the same plane. 

 So twelve of the twenty chromosomes can be demonstrated to form six 

 pairs ; the remaining eight chromosomes are so nearly of the same size 

 and form that their arrangement into pairs cannot be shown, but by 

 analogy with the others it is probable they constitute a series of four 

 pairs. One pair of the latter four probably corresponds to the pair 

 of heterochromosomes found in the prophases, but their earlier peculi- 

 arity of convoluted shape no longer persists, so they offer no means for 

 recognition. The spermatogonia, accordingly, contain each two hetero- 

 chromosomes and eighteen ordinary chromosomes. 



All these chromatin elements were longitudinally split, and became so 

 placed upon the spindle (Plate IX, fig. 9) that the daughter chromosomes 

 separate along the line of this split; fig. 10 shows an early anaphase. 

 Fig. 11 is a pole view of one of the two first spermatocytes resulting 

 from this division, and shows exactly twenty chromosomes. There- 

 fore the first spermatocyte receives a half of each of the two hetero- 

 chromosomes and of each of the eighteen ordinary chromosomes. In 

 each first spermatocyte, daughter cell of the last spermatogonic division, 

 the nucleus commences to reconstitute itself (fig. 12), The nuclear 

 membrane reasserts itself, the chromosomes commence to elongate 

 and take on more irregular contours ; but an interesting phenomenon is 

 that two of the chromosomes {n. 2, fig. 13) do not undergo these 

 changes, but remain smooth and dense; these are heterochromosomes, 

 and in all probability identical with those in the spermatogonia. At a 

 later stage (fig. 14) these unite to form a single bivalent heterochromo- 

 some (n. 2), and they retain this condition up to the time of the first 



