BIGELOW: NUCLEAR CYCLE OF GONIONEMUS MURBACIIII. 367 



chromosomes lying on its side are as intensely green." Pedicellina 

 proved to be a very favorable object for study, because the chromatin 

 retains its affinity for basic stains throughout the growth period of the 

 oocyte. And I think the evidence leaves no doubt that Dublin's con- 

 clusion, that it has no connection with the chief nucleolus, is correct. 

 Indeed, there is, so far as I know, no conclusive evidence that the chief 

 nucleolus in invertebrates ever normally contributes to the formation of 

 the chromosomes of the first cleavage spindle. 



3. The Chromatin Structures. 

 a. The Spermatogonia], Chromosomes. 



The formation of chromosomes by a linear union of smaller chromatin 

 bodies has often been observed ; but in very few cases do we know any- 

 thing about the number of such chromomeres taking part in the for- 

 mation of each chromosome. Boveri ('87) observed that in the first 

 maturation division of the egg of Ascaris each element of the tetrad con- 

 sists of six chromatin discs, while each chromosome of the germ nuclei 

 is composed of at least twice as many. Brauer ('93) was able to trace 

 these discs in more detail in the spermatogenesis of the same animal. 

 In the spireme of the primary spermatocyte there are at first about forty 

 such bodies, but before the segmentation of the spireme they are reduced 

 to ten or twelve. After the segmentation each chromatin rod contains 

 four or five, which finally fuse into a single mass. These observations 

 have led E. B. Wilson (:00, p. 302) to the conclusion that the number 

 of chromomeres is not constant for a given species. 



There are, however, a few observations tending to show that the num- 

 ber may be constant. Thus Eisen (: 00) describes the chromosomes in 

 Batrachoseps as formed each by the union of six chromomeres, and 

 Downing (: 00, : 05) finds that in Hydra each spermatogonial and each 

 somatic chromosome is formed by the fusion of four chromomeres. 



So far as I am aware, no case has ever been described in which on 

 the one hand the spermatogonial chromosomes arise by a fusion of sep- 

 arate chromomeres, while on the other hand the somatic chromosomes 

 are formed by a simple condensation of the chromatin segments, such as 

 takes place in Gonionemus. It is difficult to reach a satisfactory ex- 

 planation of this condition, but in this connection one cannot help recall- 

 ing the occurrence elsewhere of plurivalent chromosomes, as in Ascaris, 

 Artemia, and the germ cells of certain Hemiptera (McClung, : 05). 



It may be that the somatic chromosomes in Gonionemus are in reality 



