STUDIES ON CHROMOSOMES. 22J 



pairing or synapsis transpires so that nine bivalent chromosomes 

 appear in the metaphase (d, e) of this mitosis. The curved 

 body, so conspicuous an element of this stage, is regarded as a 

 bivalent chromosome formed through the fusion of the two 

 curved chromosomes of the preceding cycle. In the primary 

 spermatocyte, however, it behaves ordinarily as a single element, 

 passing undivided (e, /) to one pole of the spindle. In this way 

 two classes of secondary spermatocytes (g) are formed one with 

 and one without the' curved element. 



When the secondary spermatocytes are ready for division, 

 typically only four (7) or five (//.) chromosomes appear in the 

 equatorial plate stage. Inasmuch as the chromosomes are of as 

 large size as those of the primary spermatocytes, and since they 

 entered the secondary spermatocytes as groups of 8 or 9 respec- 

 tively, the condition found at metaphase in the secondary 

 spermatocytes, I regard as a second fusion of the eight ordinary 

 chromosomes by twos thus producing a class of cells which 

 exhibits four and one which shows five chromosomes (4 + the 

 curved element) at the division time. The double nature of 

 these large chromosomes is often indicated by a bilobed condition 

 (h, I). Their division, however, is not regarded as resulting in a 

 second reduction in the somatic number of the chromosomes. 

 The division seems rather to result in a halving of each element 

 of such a fused pair. It is not uncommon, in fact, for the 

 daughter elements each still to reveal a bilobed condition as they 

 approach the pole (j, n), or more rarely to resolve partially or 

 wholly into univalent constituents. The curved element lags at 

 the equator of the second spermatocyte while the other chromo- 

 somes are diverging toward the poles but it ultimately divides (j), 

 a moiety going to each pole. The spermatids (&) formed through 

 division of the five-chromosomed spermatocytes are represented 

 in the diagram as forming spermatozoa, those (0) descended from 

 the four-chromosomed spermatocytes are indicated as question- 

 able. The evidence on this point is adduced in a later part of 

 the paper. 



SPERMATOGONIA AND MALE SOMATIC CELLS. 

 The greatest difficulty experienced in the whole course of 

 study was in securing satisfactory preparations of the spermato- 



