Alternating Streams. xxxi 



which would become in an agamous species the first 

 polar body, is not actually extruded but may be seen as 

 a smaller cell lying close to the nucleus ; and in some 

 sexual eggs, both polar bodies are similarly retained. 



It will be apparent how simply this view of the 

 functions of the polar bodies in Cynipidae accords with 

 the facts of alternating generations. 



The fly which emerges from the gall of SpatJiegaster 

 baccarum in June is sexual, and lays an Qgg which 

 extrudes two polar bodies. The germ-plasm in these 

 polar bodies after being united with that of three sperma- 

 tozoa, is received by the embryo germ-tracks of this 

 Qg^, and when from that tgg the agamous Neuroterus 

 lentiailaris emerges in April, it is this germ-plasm 

 which forms the nuclei of the ova contained in its 

 tubes, consequently these ova can only reproduce the 

 sexual flies of SpatJiegaster bacearum. 



Again, the agamous Neuroterus lenticutaris lays an 

 ^gg w^hich extrudes one polar body. The germ-plasm 

 in this polar body is received by the germ-tracks of the 

 embryo, and, when from that ^gg the sexual SpatJiegaster 

 baccarum fly emerges, it is this germ-plasm which forms 

 the nuclear matter of the ova and spermatozoa contained 

 in its tubes, and consequently these ova and spermatozoa 

 can only reproduce the agamous fly of Neuroterus 

 lenticutaris. 



The two streams of germ-plasm are thus going on 

 independently, and are each capable of acquiring and 

 accumulating beneficial variations, so that the general 

 dictum of Weismann is correct : 'the basis of the alterna- 

 tion of generations as regards the idioplasm, must, in all 

 cases, consist of a germ-plasm composed of ids of at 

 least two different kinds, which ultimately take over 

 the control of the organism to which they give rise.' In 



