ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 249 



superimposed upon the simple system of fundamental activities (pro- 

 trusion, retraction) which is concerned with regulating the gaseous 

 exchange. J- A. T. 



Assertive Mating in Nudibranchs. — W. J. Crozier (Journ. Exper, 

 Zooh, 1918, 27, 247-02, 23 figs, and charts). A study of 148 con- 

 jugating pairs of Chromodoris zebra showed a rather high degree of 

 correlation between total length (and other dimensions) of the two com- 

 ponents of the pair. This correlation is the result of assortive mating. 

 In most cases large individuals mate successfully with large, small 

 individuals with small. The Nudibranch in question is functionally 

 hermaphrodite, and effective reciprocal insemination occurs. The 

 number of eggs deposited in a single mass varies from 2,000 to 20,000, 

 and is almost directly proportional to the length of the animal. The 

 larger animals possibly lay more egg masses in a given time than do 

 the small ones. It is consequently of advantage to the species that 

 large individuals should mate together. In this way the numbers of 

 eggs fertilized, and presumably of larv^ set free, as the result of any 

 one mating, is on the average greater than that which would be pro- 

 duced if random pairing were the rule. Moreover, by means of assortive 

 mating there is conservation of eggs, and perhaps of sperms. The 

 selective pairing may seem " purposeful," but it is clearly an automatic 

 consequence (1) of the fact that the eggs are fertilized internally, neces- 

 sitating the copulation of adults, and (2) of the fact that the size of the 

 body is not identical in all sexually mature individuals. J. A. T. 



Sex-changes in Crepidula plana. — Harley N. Gould (Proc. Amer. 

 Ass. Anatomists, in Anat. Record, 1919, 16, 149-50). This Gastropod 

 passes through a male phase, a transitional phase, and a female phase. 

 The male phase is unstable, and occurs only as the result of a_ stimulus 

 from a larger specimen of the same species. Large individuals of 

 C. fornicata have not been found to induce male development in small 

 specimens of C. plana except in a few doubtful cases. Isolation experi- 

 ments of different degrees (with G. plana) indicate that the stimulus to 

 male development is a substance given off from the bodies of the large 

 specimens, diffusible through sea-water, but very unstable. J. A. T. 



Arthropoda. 

 a. Insecta. 



Vestigial Wing in Drosophila ampelophila. — Eljier Roberts 

 {Journ. Exper. Zool., 1918, 27, 157-92, 2 pis., 3 figs.). It was found 

 that selection failed to modify the size of the vestigial wing -which marks 

 one of the varieties of this fruit-fly. It behaves as a Mendelian reces- 

 sive character when its bearers are crossed with individuals with normal 

 long wings. The offspring in the first generation have long wings, 

 while in the second generation lioth long-winged and vestigial-winged 

 individuals are produced. But these are not in the ration 3 : 1, but 

 3-95 : 1, The size and form of the vestigial wing are affected by- 

 crossing to long-winged flies. In other words the character fluctuates. 



