SAGITTA BIPUNCTATA. 15 



brane. In the prophase of the first maturation division this element 

 divides (figs. 233-234), and in metakinesis the two elements are found 

 in various positions with regard to the spindle (figs. 235-237), often as 

 conspicuous as in these figures, but sometimes concealed among the 

 chromosomes. Before the spindle for the second division forms, this 

 element divides again and one of the products goes into each spermatid 

 (figs. 238-241). 



As Sagitta is hermaphrodite, there would appear to be no question 

 of sex determination by any special chromatic element. The size of 

 the element x, its evident chromatic nature, its division before each 

 mitosis, and its presence in mitosis and in the spermatids, with the same 

 staining qualities as in the previous rest stages, certainly indicate 

 some important function, either in the whole process of spermato- 

 genesis or in the formation of the sperm-head, of which it finally 

 becomes a part. In Sagitta this element certainly can not be regarded 

 as a specialized spermatogonial chromosome, or as chromatin rejected 

 from the spireme. No such element is present in the ovogenesis of 

 Sagitta (Stevens, '03), nor has any been detected in connection with 

 fertilization. It is certain that none is present in the first segmenta- 

 tion spindle of the egg. 



GENERAL DISCUSSION. 

 THE "ACCESSORY CHROMOSOME." 



The literature bearing on the ' ' accessory chromosome ' ' of McClung, 

 the ' ' small chromosomes ' ' of Paultnier, and the ' ' chromatin nucleoli ' ' 

 of Montgomery has been fully discussed by McClung in the paper 

 entitled, "The accessory chromosome sex determinant? " ('02), and 

 will therefore be considered here only in its relation to the several 

 forms studied. The present status of the question has been well sum- 

 marized more recently by Montgomery under the heading ' ' Hetero- 

 chromosomes ' ' in the paper, ' ' Some observations and considerations 

 upon the maturation phenomena of the germ cells." 



Three theories as to the function of the ' ' heterochromosomes ' ' 

 have been advanced : (i) That of McClung that they are sex-deter- 

 minants, since in the forms which he has examined these chromatin 

 bodies occur in only one-half of the spermatozoa, and the sex-char- 

 acter is the only character which divides the individuals of a species 

 into two approximately equal groups. (2) That of Paulmier and 

 Montgomery that they are degenerating chromatin. Montgomery 

 regards them as ' ' chromosomes that are in the process of disappear- 



