A Cytological Study of the Semi-parasitic Copepod, Hersilia apodiformis etc. 41 3 



into a V or an X, or the ends may remain in contact and the pair sepa- 

 rate along their length forming a ring. The variations in the forms 

 prodnced by the separating conjugants seem to be unlimited, although 

 the ring form (r, r, Fig. 21) is most probably the primary type and that 

 which is found, ahnest without exception, in the oögenesis. In rare 

 cases, this ring form is and remains the prevaihng type in the spermato- 

 cytes, as ilkistrated by Figure 24 a— &, where there are seven ring tetrads. 

 The strepsistene threads are shorter and thicker than were the leptotene 

 threads during the conjngation period. In Hersilia, there is no twisting 

 of the conjugants about one another; and, since they separate along the 

 plane of conjugation. which is visible during the entire bouquet stage, 

 there is no evidence that an exchange of segments takes place. 



In condensing, the strepsistene threads form tetrads i) of various 

 forms, shown in Figures 23—25 (Plate V) and Figures 29 and 50 

 (Plate VI). These variations depend mostly upon the connections 

 which remain between the separating threads. Vom Rate ('92, p. 114), 

 in his study of various marine Copepods, Calanus and Euchaeta, remarks 

 upon the striking difference between the chroniosomes of the primary 

 oöcytes and those of the primary spermatocytes in the same species. 

 The same is true for Hersilia: in the oöcytes, the tetrads being in ring forms 

 or paired parallel rods (Figs. 50, 51, Plate VI), while in the spermatocytes, 

 rings, crosses and long rods with knobbed ends are found, the last nien- 

 tioned form being that most usually niet with (Fig. 29, Plate VI). 



Textfigures D—H illustrate in a semi-diagrammatic way the evolu- 

 tion of the tetrads in both sexes, the "Querkerbe" being indicated by 

 an asterisk (*). As previously stated, the double zygotene bands, before 

 separating in the strepsinema, often come to a sharp angle at or near 

 the middle, and it is quite probable that this is an indication of the linin 

 bridge which later forms the "Querkerbe". Figure D 1 shows a zygotene 

 thread beut at the middle; the components separate along their length, 

 the ends remain in contact (Fig. D 2, D 3). The chroniosomes broaden 

 and split lengthwise and one point of contact (left, Fig. D 4) broadens. 

 The further condensation to the form of a ring tetrad, such as is usuaUy 

 found in the oöcytes, is represented in Figures D 5, and DQ. The "Quer- 

 kerbe", as a light transverse line across the middle of each chromosome 



1) The terms "tetrad'- and "di-tetrad" are both iised by the author in the same 

 sense, to designate the chi-omosomes of the first spermatocyte or first oöcyte, since 

 the tetrads of Hersiha are compoced of two longitudenally spUt chroniosomes and are not 

 actually composed of eight parts as indicated in the work of Matschek. The Quer- 

 kerbe is merely an internal differentiation of each of the components of the tetrad. 



