330 SUMMARY OF CUKEENT RESEARCHES RELATING TO 



human embryo (" the Mateer ovum ") of the presomite period, the larger 

 embryo is in the primitive groove stage, with an embryonic plate 

 0-92 mm. long by 0-78 mm. broad, probably about seventeen days old. 

 The smaller embryo consists of an amniotic vesicle O'l mm. in its 

 largest internal diameter, and a yolk-vesicle • 03 mm. in its internal 

 diameter, slightly detached from it, the two being suspended in the 

 loose mesenchyme in the region of the body-stalk of the co-twin. The 

 exact character of the mechanism by which human twins are derived 

 from a single ovum is still unknown, but it seems that the embryonic 

 area or node separates into a primary mass and a secondary bud. If the 

 twin is as large as the primary embryo their chances of development in 

 an orderly manner would be equal, and this is presumably the case in 

 most instances of identical twins. Where the secondary bud is merely 

 a fragment the development would soon be arrested, and at term the 

 stunted bud would be found as a small epithelial cyst on the placenta 

 near the attachment of the umbilical cord. If the twin-bud is only 

 partly detached from the primary node there would exist the basis for 

 the various types of double monsters and teratomata. J. A. T. 



Selection among Germ-cells.— C. H. Danforth (Journ. Exper. 

 Zool, 1919, 28,' 385-411). Experiments with heterozygous fowls ex- 

 posed to alcohol vapour. The relative proportion of certain traits, 

 brachydactyly, Polydactyly, and white colour, appearing in the offspring 

 produced during periods of alcoholic treatment was compared with the 

 proportion of those traits produced in normal conditions. "The results in- 

 dicate that with at least some traits selection is possible, and that it is more 

 rigorous the more severe the treatment. It appears that germ-cells with 

 different genetic potentialities react differently. A possibility of far- 

 reaching importance is suggested : that even under normal conditions 

 the genes which determine the genetic potentiahties of a germ-cell may 

 have a real survival value for that cell, and moreover that the prevalence 

 of certain traits appearing in the adult may be in the final analysis 

 largely regulated by the advantages or disadvantages that the deter- 

 miners for such traits confer upon the germ-cells in which they chance 

 to be lodged. j. a. T. 



Winter Egg-production in Poultry.— H. D. Good ale and Grace 

 Macmullen {Journ. Exper. Zool, 1919, 28, 83-124). The mode of 

 inheritance of winter egg-production in liens has not yet been deter- 

 rnined._ It is suggested that there are two classes of winter egg-produc- 

 tion, viz. high and mediocre ; that there are two genes, both of which 

 must be present in the female zygote in order to have high production, 

 and that these are inherited according to the Mendelian dihybrid scheme. 

 But larger data are needed. J, A. T. 



Development of Guinea-Pig.— D. Carazzi {Monitore Zool. Ital., 

 1918, 29, 42-8, 115-21, 133-46, 18 figs.). An account is given of 

 the small canal in the notochord. It appears to arise by partial karyo- 

 lysis— a sort of schizocoel. It ruptures, and its dorsal portion is flattened 

 to form the notochordal plate. In that region the embryo, formerly 



