38 SUMMARY OF CURRENT RESEARCHES RELATING TO 



The mesodermic somites afford reliable criteria of the primitive meta- 

 merism of the head, but the same cannot be said of the rhombomeres. 

 The chief evidence of the metameric value of neuromeres consists in 

 their numerical correspondence with the mesodermic somites, but this 

 correspondence obtains in the head region of vertebrates for only the 

 primary brain vesicles (Neal's neuromeres I-VII), and not for the 

 secondary subdivisions of these, such as rhombomeres 1 and 2, which 

 result from the secondary subdivisions of neuromere III. Except in the 

 case of neuromeres II and III (Neal), the motor nerve relations of 

 the neuromeres do not accord with the supposition that they are meta- 

 meric structures. J. A. T. 



b. Histology. 



Cytology. — L. Doncaster (An Introduction to the Study of Cytologij, 

 Cambridge Uiiiversity Press, 1920, xiv + 280 pp, 24 pis., 31 figs.). An 

 admirably clear and scholarly introduction to cytology which will be 

 widely welcomed. It is marked by careful workmanship and sound 

 judgment. The illustrations are admirable and abundant, and there is 

 a representative bibliography. The subjects dealt with are the follow- 

 ing : The cell in general and protoplasm, the cell-organs, cell-division, 

 the centrosomes, the maturation of the germ-cells, fertilization, seg- 

 mentation, natural and artificial parthenogenesis, the cytological basis of 

 sex-determination, germ-cell determinants, the theory of the individuality 

 of the chromosomes, the mechanism of hereditary transmission, the role 

 of the cytoplasm in development and heredity. As was to be expected 

 from the author's personal investigations, prominence is given to the 

 cytological basis of hereditary transmission and of sex-determination ; 

 but to these questions a great part of the cytological research of the 

 past fifteen years has been devoted, and a judicial up-to-date exposition 

 is very timely. The book hardly deals with the physiological and 

 biochemical sides of cytology, and we venture to express the hope that 

 this self-denying ordinance will not be adhered to in subsequent 

 editions. J. A. T. 



Chromosome Dimensions. — C. F. U. Meek {Proc. Roy. Soc, 1920, 

 91, 157-65, 2 pis.). Measurements of a large number of chromosomes 

 in different types lead to the following conclusions : — The degree of 

 somatic complexity of an animal cannot be correlated with the lengths 

 of the chromosomes composing its complex, nor with the diameters of 

 these, nor with the total volume of these, nor with their number. 

 There are many different chromosomes in different types, and the 

 chromosomes composing the spermatogonial complex are not necessarily 

 identical in diameter with those composing its secondary spermatocyte 

 complex. All chromosomes composing an individual complex are not 

 necessarily of the same diameter. The tendency noted in a previous 

 communication for the chromatin volume and chromosome diameter to 

 increase from simple to complex animals must have been fortuitous. 



J. A. T. 



