RECENT ADVANCES IN SCIENCE 55 



lished fact that in certain forms, as here, no centrosome or 

 normal spindle exists at special periods of the segmentation. 

 The same author deals also with the subject of " Polyembryony 

 in Parasitic Hymenoptera : A Review" (ibid.). As its title 

 indicates, this is a summary of the facts that are known con- 

 cerning the occurrence of polyembryony, that is, the process 

 whereby one egg may produce several score of embryos, as it 

 is encountered in the parasitic Hymenoptera, small insects 

 about 1 mm. in length. About 10 eggs are laid without vital 

 injury to the host eggs. A tropho-amnion is formed as the 

 result of the division of the nuclei of the polar bodies, together 

 with a certain amount of the egg cytoplasm, and its function is 

 to nourish the inner embryonic mass. The primary embryonic 

 cell, while the tropho-amnion has been forming, has divided 

 into many germinal masses, each of which, after a large number 

 have been produced, proceeds to form a separate embryo. 

 Later, the larvae break out of their membranes and live freely 

 for a time and finally pupate inside the host's body. The re- 

 view terminates with a discussion of the significance of the 

 phenomenon. 



Other papers include : Kunkel, " The Effects of the Duct- 

 less Glands on the Development of the Flesh-flies " (Jour. Exp. 

 Zool. vol. xxvi. No. 2, July 191 8) ; Allee and Stein, " Light 

 Reactions and Metabolism in May-fly Nymphs " (ibid. August 

 191 8) ; Rich, " The Gill-chamber of Dragon-fly Nymphs " 

 (Jour. Morph. vol. xxxi. No. 2, September 191 8) ; Hampson, 

 " A Classification of the Pyralidse, subfamily Hypsotropinse 

 (Proc. Zool. Soc. Parts 1 and 2, August 191 8) ; and Cleghorn, 

 11 First Report on the Inheritance of Visible and Invisible 

 Characters in Silkworms " (ibid.). 



Vertebrata. — Johnson has studied the " Structure and 

 Development of the Sense Organs of the Lateral Canal System 

 of Selachians (Mustelus canis and Squalus acanthias) " (Jour.. 

 Comp. Neur. vol. xxviii. No. 1, August 191 8). He finds that 

 " The sensory canal system of Squalus and Mustelus comprises 

 two sets of organs known as canal organs and surface or pit 

 organs respectively. The two sets are genetically equivalent,, 

 but development in the former is carried to a further advanced 

 stage, the organs of this type forming in the adult an essentially 

 continuous column of sensory epithelium along one side of an 

 ^epithelial canal." Other papers include : Smith, " On the 



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