370 



The Journal of Heredity 



the X- and Y-chromosomes is mated abnormal character of their Y- 



to black females he should sire some chromosome, since the Y-chromosome 



tortoise males. has been thought not to be the carrier 



Spermatogenesis in tortoise males is of character genes until the discovery 



not normal according to Cutler and of "male-linked" by J. Schmidt' in 



Doncaster.^ This may be due to the fishes. 



Literature Cited 



^ Bamber, Ruth C. The Male Tortoiseshell Cat. Jour, of Gen. XII, No. 2, 1922. 



" Cutler, D. W. and L. Doncaster. On the Spermatogenesis of the Tortoiseshell Tom 

 Cat. Jour, of Gen. V, No. 2, 1915. 



^ Little, C. C. Preliminary Note on the Occurrence of a Sex-Limited Character in 

 Cats. Sci. N. S., XXXV, p. 784, 1912. 



* Winge, O. a Peculiar Mode of Inheritance and Its Cytological Explanation. Joiir. 

 of Gen. XII, 2, 1922. 



THE CAUSES OF TWINNING 



A Review 



HEREDITY is a new standpoint 

 in biology, reached through 

 more intensive and constructive 

 interest in the processes of reproduc- 

 tion. Though much more attention 

 has been given in recent years to the 

 study of the transmission of characters, 

 there is an equal or greater need of 

 understanding how the characters are 

 brought into expression in the develop- 

 ment of new individuals. Greater in- 

 terest may be claimed for the expres- 

 sion relations because they are in- 

 fluenced by the environment, while 

 transmission is independent of the en- 

 vironment. Probably on account of 

 so much debate on the question whether 

 environmental influences are entirely 

 excluded from transmission, the need 

 of more definite knowledge of the re- 

 lations of environment to expression is 

 cfjmmonly overlooked among students 

 of heredity. 



Thus it happens that a book which 

 does not profess to treat of heredity, 

 and even omits the word from its 

 index, is nevertheless to be considered 

 as an important contribution to the 

 subject. The book in question is Pro- 

 fessor Newman's "The Physiology of 



Twinning." a volume of the Science 

 Series of the University of Chicago.' 

 The writing and printing are well done, 

 the treatment is not over-technical, and 

 the reader is assisted by about seventy 

 illustrations very skilfully drawn by 

 Mr. Kenji Toda. The general interest 

 of the book might not be suspected 

 from the limited subject, but the treat- 

 ment leaves no doubt that twinning is 

 an important "lead" toward a better 

 understanding of some of the funda- 

 mental problems of reproduction. 



Development of two or more em- 

 bryos from one egg is considered as 

 the only genuine twinning, and this 

 phenomenon is traced through all of 

 the principal groups of animals, with 

 special consideration, of course, for 

 the armadillos. With these animals 

 twinning is a regular occurrence and 

 is supposed to be connected with a de- 

 lay of development at the period of 

 gastrulation, which Professor Newman 

 considers as a general cause of twin- 

 ning, and subject to environmental in- 

 fluence. The process of twinning is 

 conceived fundamentally as a form of 

 fission, analogous with dichotomous 

 branching of plants. Fasciation is 



' The Physiology of Twinning, by Horatio Hackett Newman, Professor of Zoology, 

 University of Chicago. Pp. 230, xii. Price $1.75. The University of Chicago Press. 192S. 



