223 



Sex-determination in Hydatina, with Some Remarks on 

 Parthenogenesis. 



By E. C. Punnett, Fellow of Gonville and Caius College and Balfour Student 

 in the University of Cambridge. 



(Communicated by Adam Sedgwick, F.R.S. Received May 15, — 

 Eead June 28, 1906.) 



[Plate 11.] 



Introductory. — Those writers who hold that the sex of an animal organism 

 can be affected by a modification of external conditions during the period 

 of development have of late years supported their case by reference to the 

 parthenogenetic rotifer, Hydatina senta. Upon certain of the facts of 

 reproduction there is a general consensus of opinion among those who have 

 made a study of this species. Briefly summarised it is as follows : — 



Three kinds of ova are produced by this rotifer, viz. : (1) parthenogenetic 

 ova which develop into ? 's ; (2) parthenogenetic ova of smaller size which 

 develop into j 's ; and (3) fertilised eggs which always develop into ? 's. 

 Any given ? lays but one of these three classes of eggs during her lifetime. 

 We may, therefore, distinguish three kinds of ? 's by the eggs they lay, 

 viz. : (a) $ 's which produce ? 's parthenogenetically, or thelytokous ? 's ; 



(b) ? 's which produce <$ 's parthenogenetically, or arrenotokous ? 's ; and 



(c) the layers of fertilised eggs. Both thelytokous and arrenotokous ? 's can 

 be impregnated by the On the former impregnation has no effect. In 

 the case of the arrenotokous ? 's impregnation has no effect, unless it occurs 

 during the first few hours after hatching. When this takes place such a ? 

 produces fertilised eggs instead of the <$ eggs which it would otherwise have 

 laid. The layers of fertilised eggs are, therefore, arrenotokous ? 's, which 

 owe their special nature to the circumstance of having been impregnated 

 by the S during the earlier stages of their growth. And in this connection 

 it is interesting to recall Lenssen's observation that the $ egg extrudes a 

 single polar body, whilst the ? egg extrudes none.* 



Though the thelytokous ? 's cannot be fertilised they are able to give rise 

 parthenogenetically to arrenotokous ? 's as well as to other thelytokous ? 's. 

 The proportion of arrenotokous ? 's so produced is subject to considerable 

 variation, and to connect these variations with external conditions has been 

 the object of those who have bred these animals with a view to throwing 

 light upon the vexed problem of sex-determination. 



* ' La Cellule,' vol. 14, 1898. 



