A Cytological Study of the Semi-parasitic Copepod, Hersilia apodiformis etc. 439 



threads assume a regulär polar arrangement, the leptoteiie bouquet. 

 Optical sections show that the loops are most probably present in 

 unreduced number. 



5. There is a period of conjugation, It is characterized by the side- 

 by-side union of the threads in pairs. During this period the polar orien- 

 tation of the loops is partially lost. The fusion of the nucleoli and the 

 imion of the "cyto-plasmosomes" to form a "nuclear cap" occur in the 

 transitional period. 



6. The zygotene bouquet foUows the period of conjugation. There 

 is again a definite polar orientation of the chromatin loops, which are 

 double and present in reduced number. The large "nuclear cap", with 

 the nucleolus lying usually below it in the nucleus, is characteristic of 

 this stage. 



7. Hersilia has no "confused stage" in the spermatocytes, the 

 zygotene threads being always visible. At the end of the bouquet 

 stage they lose their polar orientation and open out along the plane 

 of conjugation. The nucleolus gradually diminishes in size and dis- 

 appears. The "nuclear cap" contracts to a hollow sphere; and, in the 

 dividing spermatocytes, the only traces of it are small, deeply staining 

 fragments in the cytoplasm. 



8. In the spermatocytes long rods, short rods, ring- and cross-shaped 

 tetrads are formed. In the oöcytes are either rings or double parallel 

 rods. The primary oöcyte metaphase shows twelve tetrads: eleven with 

 "Querkerbe" (in at least two, asymmetrically placed), and one small 

 tetrad without "Querkerbe". In the first spermatocyte spindle thirteen 

 chromosomes are to be counted in about three-fourths of the cases; the 

 others show twelve. This Variation arises from the pairing or non-pairing 

 of two hetero-chromosomes. 



9. In the first spermatocyte mitosis the chromosomes separate re- 

 ductionally, whole chromosomes with "Querkerbe" passing to the poles. 

 The two hetero-chromosomes lag, but normally one goes to each second 

 spermatocyte. In exceptional cases one may be halved by the division 

 of the cytoplasm into sister ceUs. Such distribution of the chromosomes, 

 as this would occasion, might account for rare cases of hermaphroditism 

 in Hersilia. 



10. In the interkinetie period the longitudinally split chromosomes 

 assume the forms of X's, Y's, or V's. 



11. In the second spermatocyte metaphase the longitudinally divided 

 chromosomes lie with their long axes in the equator. Twelve chromo- 



