A Cytological Study of the Semi-parasitic Copepod, Hersilia apodiformis etc. 421 



somes of Cyclops in size, miglit iiaturally be expected to be different 

 from tlie larger chromosonies ; but it is one of the larger individuals and 

 often shows a "Querkerbe", especially when seen in lateral view in the 

 second maturation division, with its halves joined to the two autosome 

 groups, and in contact with each other at one end (Fig. 40). 



Figure 41 represents a spermatid shortly after the division of the 

 cytoplasni of the second spermatocyte. The hetero-chromosonie lies 

 beside the telophase chromosonies, which lose their identity in the forma- 

 tion of the spermatid nucleus. As the nucleus increases in size, a light 

 area appears in the cytoplasm and at one side of this area the hetero- 

 chromosonie is located (Fig. 42). Such spermatids as that shown in 

 Figure 43 indicate that the hetero-chromosonie is included in the 

 growing nucleus, the nuclear membrane forming last in the region 

 of the light area. A similar case to this was found in the Coleop- 

 teran Haltica by Miss Stevens ('09). In the further metamorphosis 

 of the spermatid. no evidence as to the presence of the hetero- 

 chromosome could be found. 



VIII. The Chromosomes of the Cleavage Cells and Primordial Germ Cells. 



A. Cleavage Cells. 



In the cleavage cells of Hersilia twenty-four chromosonies can be 

 counted. Figure 52 pictures a metaphase (equatorial) plate of the third 

 cleavage stage. The chromosomes are of varied form, long, and niuch 

 larger than those of the spermatogonia and oögonia. In the rod forms 

 the "Querkerbe" could sometimes be plainly seen; but the V-forms 

 with only a thin Strand connecting the two arms at the apex, were more 

 numerous than rod forms, and this thin Strand is doubtlessly equivalent 

 to the "Querkerbe". Agar ('12) found these V-shaped chromosomes 

 with connecting bridge to be the niost usual type in the embryonic tissues 

 of Lepidosiren. In Figure 52 are also to be seen those chromosomes 

 in which the "Querkerbe" is not located at the middle and in form they 

 generally look like "L's". The small single pairs are difficult to recognize, 

 although most probably they are the shortest pair of rods, lying in the 

 Upper left of the figure. In the further development of the embryo, the 

 chromosonies retain the same form exhibited in the early cleavage stages, 

 but are smaller and more difficult to count. In the blastula (correspond- 

 ing to the stage following that shown in Figure 53) twenty-four chromo- 

 somes could be counted in the dividing blastomeres. The results on 

 Hersilia agree with those of Amma '(11) for many Copepods, as regards 



