ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 757 



Diurnal Migrations of Calanus finmarchicus.* — 0. 0. Esterly finds 

 that this Copepod is most abundant at the surface about 7 or 8 p.m. 

 during June and July. It has begun to leave the surface at midnight, 

 and is then most abundant at a depth of 5 or 10 fathoms. During the 

 day (6 a.m.-6 p.m.) it is most abundant at about 200 fathoms. To- 

 wards evening there is an upward movement of a large part of the 

 population. 



The cause of the migration is probably the effect of light upon the 

 organism, though it does not seem that a definite movement towards a 

 dim light or away from a stronger one can be at the basis of this 

 behaviour. The effect of light upon the geotropism of the animals is 

 probably the main factor involved. 



Ostracods from Madeira. f — G. Stewardson Brady describes a number 

 of new species of Bairdia, Cythere, Cythereis, Cytherura, Paradoxostoma, 

 etc., from Madeira. The collection is also interesting as extending the 

 known range of several species from the European and North Atlantic 

 areas much further southward, though not quite into the tropical zone. 



Sex-determination in Daphnids.J — R. "Woltereck has experimented 

 with Hyalodaphnia, and has reached the following conclusions. External 

 conditions (temperature, food, chemical reagents), may sometimes exert 

 a distinct influence on the sex of the developed ovarian ova (' induction ') 

 as well as on the ova to be subsequently formed (pre-induction). Such 

 influences act on an internal nexus of causes, and it is noteworthy that 

 they sometimes have no effect at all. Susceptible and non-susceptible 

 periods may alternate. Thus there may be pure parthenogenesis, sus- 

 ceptibility or lability, well defined bisexuality, susceptibility or lability, 

 pure parthenogenesis, and so on. 



The internal causes do not depend simply on the degree in which the 

 ova are equipped with plasma-substances or with heterochromosomes 

 (assimilation-chromatin). Nor does it appear that the " Kern-plasma 

 relation," resulting from continuous parthenogenesis or from external 

 conditions, effects the determination of sex. 



The author's experiments lead him to the view that there are in each 

 ovum competing sex-substances, one of which becomes active as the egg 

 matures, while the other remains latent. The latency is due to some- 

 thing preventing the substance from becoming active. This inhibition 

 may occur at two different periods. It may occur shortly before the 

 ovum leaves the ovary. The definitive determination of sex then takes 

 place (" induction of sex by hindrance of activation"). Secondly, it may 

 take place much earlier (a) in the undifferentiated germ-layer of the 

 ovary ; (b) in the gonad-primordium of vhe winter-egg (embryo) ; 

 (c) in the ripening female egg. During these periods a change seems 

 to take place or to begin to take place in the sex-substances, which may 

 be called their " maturation." This change tends to bring about in the 

 sex-substances the faculty of afterwards being activated. If this process 

 during one of the critical periods is hindered in its realization in one of 



* 



Internat. Rev. Hydrobiol., iv. (1911) pp. 140-51. 

 t Proc. Zool. Soc. (1911) pp. 595-601 (3 pis.). 

 % Internat. Rev. Hvdrobiol., iv. (1911) pp. 91-128 (6 figs.). 



