Mat 27, 1921] 



SCIENCE 



503 



the crops on whicli ttey occur and are sub- 

 divided into diseases of major and minor 

 importance. This arrangement is especially 

 serviceable to those who are not specialists on 

 plant diseases. The descriptions of the symp- 

 toms are brief, clear and very readable. There 

 is no attempt whatever to discuss the organ- 

 isms which are the causes of these diseases 

 but references are given to some of the more 

 important publications. Each disease is desig- 

 nated by its common name; the scientific 

 name for both the imperfect and the perfect 

 stages, where known, are placed in paren- 

 thesis. The book also contains chapters on 

 the history of the subject, damages due to 

 plant diseases, prevention and cure, general 

 diseases which attack a large number of 

 crops, fungicides and soil disinfection. The 

 chapter on cost of spraying which was in the 

 first edition is very properly omitted since 

 this is a varying factor dependent on cost of 

 materials and labor. 



The work is intended primarily as a text- 

 book and it will prove of great service to all 

 teachers of plant pathology. Possibly its 

 greatest value lies in the brief, clear descrip- 

 tions which are of such great importance in 

 making diagnoses of diseases in the field. The 

 student of mycology will also find it an 

 important supplement for his work on eco- 

 nomic forms. The horticulturists, nursery- 

 men, county farm demonstrators, progressive 

 farmers and in fact all others who are 

 interested in the applications of agriculture 

 will find it an extremely useful reference 

 book. 



The mechanical make-up of the work is 

 good except for the crowded arrangement of 

 the bibliography which would lead any one 

 who uses it to fear that the supply of paper 

 is exhausted- 



Mel T. Cook 

 , New Jersey AGEictrLTiiKAL 

 Experiment Station 



SPECIAL ARTICLES 



THE Y-CHROMOSOME IN MAMMALS 



The majority of workers on mammalian 

 spermatogenesis have described the sex-chro- 



mosome as being of the X-0 type but recent 

 investigations in this field by the author in- 

 dicate that the X-Y type of chromosome may 

 be more common than is generally thought. 



In the opossum,^ an animal for which the 

 X-0 type of sex-chromosome has been de- 

 scribed, the writer finds a typical X-Y sex- 

 chromosome complex. Both the X and Y com- 

 ponents may be recognized in si)ermatogonial 

 and somatic divisions because of their dis- 

 tinctive size. In the first maturation division 

 the X and Y elements s^r^ate apart to 

 opposite poles of the cell, and in the second 

 maturation division both divide equationally. 

 Hence half of the sperm carry an X and half 

 carry a Y chromosome. 



The diploid chromosome number for both 

 the male and female opossum is 22, and not 

 17 or 24 as concluded by previous investi- 

 gators. 



In the testes of both the white man and the 

 n^ro I have found in the first spermatocytes 

 a chromosome pair which is similar in appear- 

 ance and behavior to the X-Y chromosome of 

 the opossum. The two members of this pair, 

 in the human, representing the X and Y 

 components, are unequal in size; they segre- 

 gate apart in the first maturation division just 

 as in the case of the opossum. 



It will be of general interest to biologists 

 to know that the diploid number of chromo- 

 somes for man is very close to the number 

 (47) given by "Winiwarter.^ In my own 

 material the counts range from 45 to 48 ap- 

 parent chromosomes, although in the clearest 

 equatorial plates so far studied only 46 chro- 

 mosomes have been found. Before a final 

 conclusion is made on the exact number it is 

 desired to make a careful study of a large 

 number of division plates. There can be 

 absolutely no question, however, but that the 

 diploid number of chromosomes for both the 

 white man and the negro falls between 45 and 

 48. With the X-Y type of sex-chromosome we 



1 The -writer 's work now In press. 



2 Winiwarter, H. von, 1912, Arch, de Biol., 

 Vol. 27. 



