180 Reproduction in Siinocei)halus vetulus 



but in connection with xariout;; problems of heredity. As, howcvor, the 

 experiments have been extensive, and as one line in particular was bred 

 for 46 successive generations, they are able incidentally to afford very 

 instructive information upon the sexuality question. 



It would be useless to review completely the bulky literature dealing 

 with rejjroduction in Cladocera, and for this I refer the reader to the 

 discussions and literature lists in the works of Papanicolau, Woltereck 

 and Hertwig. It is well, however, to mention a few researches dealing 

 specially with S. vetulus. 



This species was one of those dealt with by Weisraann in 1879, and 

 on which his theory quoted above was based. 



Issakowitsch (1908), who experimented with this among other species, 

 comes to the conclusion that it has an inherent reproductive cycle, the 

 duration of which is not, however, independent of external conditions. 

 For the " cycle " is due to an increasing " depression " of the germ cells 

 (in the nucleo-cytoplasm relation sense) which in the early stages can 

 be alleviated by various external factors, but at last gets to a stage when 

 nothing but conjugation will save it from death. 



Kuttner (1909) supported Weisinann's extreme view. S. vetulus was 

 one of the species experimented with. 



We must consider Papanicolau 's experiment (1911) rather more 

 closely as it was carried out in great detail and thoroughness, and as 

 the conclusions have been accepted by R. Hertwig and used by him 

 in support of his theory of " depression " periods leading to sexual 

 conjugation. 



Papanicolau's main experiment dealt with a line bred from an 

 ex-ephippio female at room temperature, and gave the following chief 

 results. 



1. The line bred parthenogenetically for twenty generations, sexual 

 and degenerate forms getting more and more numerous, the twentieth 

 generation consisting entirely of males and degenerated animals, and 

 parthenogenesis then ceased altogether. 



2. Sexual individuals began to appear in late broods of early 

 generations (e.g. in the tenth brood produced by generation 2, in the 

 eighth brood produced by generation 3) and appeared in earlier and 

 earlier broods in later generations, till they occurred in the second 

 brood of generation 17 and first of generation 20. 



3. A gi'eat many weak and degenerate forms appeared, often 

 abnormal and dying before birth. The appearance of these degenerate 

 forms accompanied that of the sexual ones, and the same phenomenon 



