1906.] 



Sex-determination in Hydatina. 



229 



fection of analysis, and (4) of ? 's which, according to Maupas* produce three 

 arrenotokous ? 's out of every four. Beyond the fact that (1) may arise from 

 (3) we know nothing of the inter-relations of these various types. 



Whether or not there are zygotically distinct types of $ 's we are at present 

 quite ignorant. The possibility might, however, be tested by a system of 

 fertilisation experiments and subsequent breeding from the ? 's hatched. 



The Nature, of Parthenogenesis. — Any interpretation of the facts which is 

 based upon inherent differences in the zygotic constitution of individual ? 's 

 must necessarily lead us to modify our views as to the nature of partheno- 

 genesis. A given ? can be constructed on but a single zygotic plan. 

 Consequently if she produces eggs without the formation of polar bodies, as 

 Lennsenf has suggested, these eggs, provided they contain the somatic number 

 of chromosomes, ought all to possess the same zygotic constitution and the 

 same zygotic properties. But if we assume, and the assumption seems 

 warranted by facts, that external influences are without effect, we must then 

 infer that a ? which produces both arrenotokous and thelytokous ? 's is 

 producing two entirely different kinds of egg. In the light of recent 

 Mendelian research such a condition implies a process of segregation which 

 is probably bound up with a reduction in the somatic number of chromo- 

 somes, and the subsequent restoration of that number by the union of two 

 gametic cells each containing the reduced number. Though without 

 evidence either way we have no reason to suppose that the somatic number 

 of chromosomes varies in Hydatina during successive generations. We are, 

 therefore, led to suppose that, somewhere antecedent to the formation of an 

 ovum, there is a gametogenic process, and that the recombinations which 

 take place among the gametes so formed determine the proportion of 

 arrenotokous $ 's. In other words, it seems not inconceivable that the 

 thelytokous ? 's are really hermaphrodite, though the $ gametes may not 

 exhibit the orthodox form of spermatozoa.^: Such a view would account for 

 the observed absence of polar bodies in the $ eggs, for it must be supposed 

 that the process of reduction and fertilisation takes place before the accumu- 

 lation of yolky material. The ? eggs would on this view be comparable, not 

 to the parthenogenetic $ eggs, but to $ eggs which had undergone fertilisa- 

 tion with the formation of " winter " eggs. 



On the other hand it must not be forgotten that, although unable to 



* ' Comptes Eendus,' October 6, 1890. 

 t ' La Cellule,' vol. 14, 1898. 



1 In this connection may be mentioned the interesting discovery of Brauer, that in the 

 parthenogenetic egg of Artemia a second polar body is extruded, but immediately re-enters 

 the ovum and fuses with the ? pronucleus. 



