392 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Sorex araneus and probably also in the upper jaw of Neomys. Perhaps 

 this bridges the gap between the Polyprotodonta and the Monodelphia. 

 Another discovery is the presence of a tooth-rudiment belonging to the 

 milk-dentition situated between the upper fourth premolar and the first 

 molar in Grocidmra rus8ula. There are other rudimentary antemolars, 

 proving that the Soricid^e once had a full set of permanent teeth. No 

 doubt need remain as to the two dentitions of Soricidge : there is a milk- 

 dentition as well as the permanent dentition. Indeed, besides these 

 dentitions there are traces of a third in Sorex araneus, that is to say of a 

 pre-lacteal dentition. As regards tooth-development the three genera, 

 Sorex, Neomys, Grocidura, form a series, Sorex being the most primitive 

 and Grocidura the most advanced form, Neomys holding an intermediate 

 position. 



Development of Vertebrate Eye.* — H. Spemann has made experi- 

 ments on the embryos of Rana, Bombinaior, and Triton with reference 

 to the development of the eye. For some Vertebrates it has been proved 

 that certain cells of the skin can develop on their own account, apart 

 from an optic cup, into a highly differentiated lens. In other cases 

 there is not this capacity of independent development, though there is 

 an attempt at proliferation on the part of the cells in question. It seems 

 likely that the optic cup has some power of prompting epidermis cells,. 

 which would not otherwise form a lens, to lens- formation. In some 

 forms the independence of lens-forming cells is prominent ; in others 

 the influence of the optic cup is more noteworthy. It is very unlikely 

 that the influence of the optic cup on lens-formation is merely mechanical. 

 It is more likely that the optic cup exerts some specific chemical influence 

 on the epidermic cells. 



Heterochromosomes and Sex-determination. t — J. Gross has tested 

 the conclusion that there are special sex-chromosomes. He has especially 

 investigated Syromastes, Pyrrhocoris and Anasa, and he has studied the 

 facts recorded in regard to many types of insects. His conclusion is 

 against the theory that there are special accessory chromosomes deter- 

 mining sex. 



Action of Radium Rays on Spermatozoa. $ — Giinther Hertwig has 

 subjected the spermatozoa of Parechinus miliar is to radium rays, and 

 then used them in fertilizing normal ova. The results appear to prove 

 that the detrimental effects of the radium influence are transferred 

 to the ovum-chromatin by the nucleus of the spermatozoon, and also 

 that an intense radiation brings about in the sperm-chromatin an 

 incapacity for increase. 



Absorption of Frog's Eggs.§ — L. Burkardt has studied the absorp- 

 tion of the unlaid eggs of Rana escuhnta. It is initiated by the dis- 

 ruption of the zona pellucida and the increase of the granulosa cells. 

 After the disappearance of the zona pellucida the granulosa cells 



* Zool. Jahrb., xxxii. (1912) pp. 1-98 (6 pis. aud 15 figs.). 



t Zool. Jahrb., xxxii. (1912) pp. 99-170. 



t Arch. Mikr. Anat., Ixxix. (1912) Heft 4, Abt. ii., pp. 201-11 (12 pis. and 9 figs.) 



§ Arch. Mikr. Anat.,lxxix. (1911) Heft 1, Abt, ii., pp. 1-40 (3 pis. and 1 fig.). 



