2/2 REDUCTION OF THE CHROMOSOMES 



In Pentatoma the number of chromosomes in the spermatocyte is fourteen. 

 During the final anaphases of the last division, one of the fourteen daughter-chromo- 

 somes assumes a different staining-capacity from the others, and becomes a '• chro- 

 matin-nucleolus " which fragments into several smaller bodies during the ensuing 

 resting-stage. During each of the succeeding spermatocyte-divisions appear seven 

 chromosomes and a single small chromatin-nucleolus, and both of these kinds of 

 bodies are halved at each division, so that each spermatid receives seven chromo- 

 somes and a single chromatin-nucleolus.^ In Xipliidiiiin a body called by McClung 

 the "accessory chromosome," and believed by him to correspond to the '' chromatin- 

 nucleolus " of Pentatoma, appears in the early prophases of the last spermatogonium- 

 division while the remaining chromatin still forms a reticulum. In the equatorial 

 plate this lies outside the ring of chromosomes, but divides like the latter. The 

 same body appears in the ensuing resting-stage, and during both of the spermatocyte- 

 divisions. In these it lies, as before, outside the chromosome-ring, and differs 

 markedly from the other chromosomes, but divides like the latter, each of the halves 

 passing into one of the spermatids, where it appears to form an important part of the 

 sperm-nucleus. 



Despite the peculiarities described above, the chromatin, as a whole, seems to be 

 equally distributed in both Pentato)iia and Xiphidiiun. In Anasa, however, Paul- 

 mier's studies ('98, '99), made in my laboratory, give a result agreeing with that of 

 Henking, and suggest some very interesting further questions. The spermatogonia- 

 nuclei contain two nucleolus-Iike bodies, and give rise to twenty-two chromosomes, 

 of which two are smaller than the others (Fig. 126). In the first spermatocyte-divi- 

 sion appear eleven tetrads. Ten of these arise from rings like those of Gryllotalpa, 

 etc. The eleventh, which is much smaller than the others, seems to arise from a 

 single nucleolus-like body of the spermatocyte-nucleus, and by a process differing 

 considerably from the others. All of these bodies are halved to form dyads at the 

 first division. In the second spermatocyte-division (Fig. 127) the larger dyads 

 divide to form single chromosomes in the usual manner. TJie small dyad, however, 

 fails to divide, passing over bodily into one of the spermatids. In this case, there- 

 fore, half of the spermatids receive ten single chromosomes, while the remainder 

 receive in addition a small dyad. 



A comparison of the foregoing results indicates that the small tetrad (dyad) corre- 

 sponds to the extra chromosome observed by Henking in Pyrrochoris, and perhaps 

 also to the '"accessory chromosome" oi Xiphidiiun. Whether it corresponds to the 

 " chromatin-nucleolus " of Pentatoma is not yet clear. The most remarkable of 

 these strange phenomena is the formation of the small tetrad, which seems to be a 

 non-essential element, since it does not contribute to all the spermatozoa. Paulmier 

 is inclined to ascribe to it a vestigial significance, regarding it as a "degenerating" 

 chromosome which has lost its functional value, though still undergoing in some 

 measure its original morphological transformation ; in this connection it should be 

 pointed out that the spermatocyte-nucleolus, from which it seems to be derived, is 

 represented in the spermatogonia by two such nucleoli, just as the single small tetrad 

 is represented by two small chromosomes in the spermatogonia-mitoses. The real 

 meaning of the phenomenon is, however, wholly conjectural. 



E. The Early History of the Germ-nuclei 



There are many peculiarities in the early history of the germ- 

 nuclei, both in plants and animals, that have a special interest in con- 



^ On this latter point Montgomery's observations do not seem quite decisive. 



