Mr Doncaster, Development of the Unfertilized Egg, etc. 103 



On the Maturation and Early Development of the Unfertilized 

 Egg in certain Sawflies (Tenthredinidae). By L. Doncaster, 

 M.A., King's College*. 



[Received 24 February 1905.] 



In a former note (Proc. Camb. Phil. Soc. Vol. xn. 1904, p. 474) 

 I gave an account of the maturation and fate of the polar bodies 

 in Nematus ribesii, a sawfiy in which unfertilized eggs yield males. 

 The present note summarizes my investigations on other species 

 of the family. 



The species observed, in addition to N. ribesii, are Nematus 

 pavidus and N. lacteus, Poecilosoma luteolum, Croesus varus and 

 Hemichroa rufa. Of these species, N. pavidus is known to produce 

 males from virgin eggs ; iV. lacteus lays eggs readily when not 

 impregnated, and these develope to young larvae, but I know of 

 no case in which they have been reared to adult flies. The species 

 resembles N. pavidus so closely, however, that it probably belongs 

 to the male-producing group. In N. lacteus the maturation and 

 fate of the polar bodies was very similar to that described for JSf. 

 ribesii; the second polar mitoses gave rise to four similar nuclei 

 lying near the dorsal edge of the egg, of which the innermost is 

 the egg-nucleus, the three outer the polar nuclei. The egg- 

 nucleus sinks into the yolk and begins to divide to form the 

 embryo, the outermost polar nucleus degenerates, and the two 

 inner nuclei move together until they come into close contact. 

 It appears however that the fusion is less complete than in iV. 

 ribesii, and the two nuclei break up into chromosomes, giving two 

 groups lying side by side, each containing 8 chromosomes. One 

 of these groups usually rapidly disappears, the other persists for 

 some time longer. The series of N. pavidus was unfortunately 

 not complete, but the stages obtained make it almost certain that 

 the course of events is similar to that described in N. lacteus ; it 

 appears therefore that both these species are essentially similar to 

 N. ribesii in their early development. 



Poecilosoma luteolum is known to produce only $s from un- 

 fertilized eggs, and can be bred for several generations without 

 any males appearing. The maturation mitoses take place as 



* A full paper with figures and discussion will be published elsewhere. 

 VOL. XIII. PT. II. 8 



