20 GENETICS [BoT. Absts., Vol. X, 



the same as the form which has been described and named L. nelsoni Henshaw. — Both sets 

 of hybrids in the juvenile stage more closely resemble the darker parent. — L. J. Cole. 



106. McRosTiE, G. P. The immunization of plants. Sci. Agric. [Canada] 1: 122-124. 

 1921. — The present paper, read before the Quebec Society for the Protection of Plants, dis- 

 cusses the general ideas of selection and hybridization to secure disease-resistant plants. — 

 B. T. Dickson. 



107. Malone, J. Y. Spermatogenesis of the dog. Trans. Amer. Microsc. Soc. 37: 97-110. 

 S pi. 1918. — The spermatogonia show 21 chromosomes. The leptotene thread apparently 

 undergoes parasynapsis. The X chromosome stands apart as a compact dark-staining mass. 

 Ten bivalent and an X chromosome appear in the metaphase of the primary spermatocyte. 

 The X chromosome passes undivided to one pole. The secondary spermatocytes show 10 

 and 11 chromosomes, respectively. In spermiogenesis the centrosome gives rise to the end- 

 knob, axial filament, and the posterior centrosome; the sphere substance to the acrosome; 

 and the spermatosphere to the sheath of the middle piece. Measurements of mature sperma- 

 tozoa show a bimodal curve. — M. F. Guyer. 



108. Morgan, T. H., A. H. Sturtevant, and C. B. Bridges. The evidence for the 

 linear order of the genes. Proc. Nation. Acad. Sci. [U. S.] 6: 162-164. 1920. — This paper is 

 the final answer to the criticisms of Castle of the theory of the linear order of the genes in 

 the chromosome, and to his suggested 3-dimensional chromosome model. The authors 

 emphasize the proof already cited that the linear order is shown by building up the whole 

 chromosome by combining "distances" so short that no double-crossover classes appear. 

 "The purpose of the chromosome maps is two-fold: 1st, to give the sequence of the loci, and 

 2nd, to indicate by the relative spacing of the loci the crossover values most likely to coincide 

 with the results of future experiments." In order to discover the 1st point it is necessary to 

 use data in which all loci however widely separated are followed in a single experiment, while 

 the latter point can be determined best by the use of all available data including intermediate 

 points. It has already been shown why the two do not necessarily correspond, yet Castle 

 states that the authors reject "nearly 99 per cent" of their data in the case of the yellow, 

 bifid section of the map, and reverse the method in constructing their model. It is also stated 

 that there is nothing impossible in crossing over in excess of 50 per cent. The authors believe 

 that all of Castle's objections have been met, and that his 3-dimensional scheme does not fit 

 the data. — H. H. Plough. 



109. MuLLER, H. J. Are the factors of heredity arranged in a line? Amer. Nat. 54:97-121. 

 4 fig. 1920. — The author shows that Castle's objections to the linear arrangement of genes 

 in chromosomes, and his substitute non-linear 3-dimensional models are invalid, since they 

 involve, among others, the following gratuitous or erroneous assumptions: (1) Shapes and 

 sizes of organic molecules; (2) that double or triple crossover does not occur; (3) that 

 data from unrelated experiments are comparable; (4) that both small and large frequencies 

 of separation can be represented by straight lines in some single consistent model; (5) 

 that proportionate representation of separation frequencies is compatible with polarized 

 breaks in linkage; (0) that map-distances greater than 50 units must connote separation fre- 

 quencies greater than 50 per cent; and (7) that coincidence can be left unconsidered. It is 

 shown that, mathematically considered, genes are arranged in a bipolar fashion, each linked 

 directly to only 2 others, those lying to the right and to the left in a line all parts of which are 

 straight, — a relation that, physically considered, requires a material connection of gene to 

 gene in chain formation. — Calvin B. Bridges. 



110. Nachtsheim. [German rev. of: Metz, C. W. Chromosome studies in the Diptera. 

 I. A preliminary survey of five different types of chromosome groups in the genus Drosophila. 

 Jour. Exp. Zool. 17: 45-56. 28 fig. 1914. Idem. II. The paired association of chromosomes 

 in the Diptera and its significance. Jour. Exp. Zool. 21: 213-262. 8 pi. 1916. Idem. III. 

 Additional types of chromosome groups in the Drosophilidae. Amer. Nat. 50: 587-599. 1916.] 

 Arch. Zellforsch. 15: 310-312. 1920. 



