RECENT ADVANCES IN SCIENCE 233 



been carried out in this group of the animal kingdom. In 

 " The Regeneration of Triangular Pieces of Planaria maculata, 

 a Study in Polarity" (Journ. Exp. Zool. vol. xxv. Feb. 191 8), 

 Olmsted describes a series of experiments made on the regenera- 

 tive power of this flat-worm. The animals were cut in various 

 ways, and under certain conditions it was found possible to 

 bring about the regeneration of a new head at right angles 

 to the former median axis. Except under these fairly limited 

 conditions there is undoubtedly a marked tendency for the 

 original polarity to be retained. Copeland, in his experiments 

 on " The Olfactory Reactions and Organs of the Marine 

 Snails Alectrion obsoleta (Say.) and Busy con canaliculatum 

 (Linn.) " (ibid.), found that the olfactory organ in Alectrion 

 was undoubtedly situated within the mantle cavity. To 

 complete the inquiry, however, a larger species was investi- 

 gated, and it was found that if the osphradium was destroyed 

 the sense of smell was also eliminated and did not return 

 until, at any rate, a partial regeneration of that organ had 

 occurred. The siphon which is moved from side to side during 

 progression introduces the olfactory stimulus into the mantle 

 cavity and so enables the animal to discern on which side the 

 cause of the smell lies, and so, if it be food, the animal can move 

 towards it. It was found possible, by taking advantage of 

 this, to lead the snail about the aquarium. " The reactions 

 to light and gravity in Drosophila and its mutants " have been 

 investigated by McEwen (ibid.). The author finds that the 

 young female is slightly more positively phototrophic than 

 the young male, but after nine days this difference has prac- 

 tically disappeared, and also interestingly enough that certain 

 mutants, whose wings are absent or defective, show no photo- 

 tropism in the same way as normal flies whose wings have 

 been removed. Lastly, this indifference to light appears as a 

 sex-limited character in the nutant known as " tan." 



The somewhat striking results of Martini, who reported 

 that in Hydatina, a Rotifer, the numbers of cells in the various 

 organs were constant, and so the total number in the whole 

 animal was also constant, have been reinvestigated by Shull 

 in " Cell Inconstancy in Hydatina senta" (Journ. Morph. vol. 

 xxx. March 191 8). It was found that there is a slight but not 

 very marked variation in the number of cells in each organ, 

 and also in the number of nuclei in the syncytial yolk gland. 



