ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 125 



environment act on the egg after fertilization. Some of the experimental 

 work on secondary sex-characters suggests the possibility of the sex of 

 an embryo being modified after fertilization by an alteration in the 

 physiological conditions. 



In regard to secondary sex-characters, Doncaster suggests, as others 

 have done, that each sex may have the potentiality of producing either 

 male or female sexual characters, and that whether the one or the other 

 set of characters appears depends on the particular kind of metabolism 

 of the tissues concerned. It is important to distinguish between factors 

 for sexual characters and for sex-determination. The inherited factors 

 for the secondary sexual characters may be present in each sex, and the 

 sex-determining factor may decide which shall appear. The problems 

 of hermaphroditism and gynandromorphism are discussed in a special 

 chapter. 



The general conclusions of this interesting and luminous book may 

 be briefly indicated. Many facts point to the reality of a sex- 

 determining factor resident in the sex-chromosomes and inherited like 

 any other Mendelian character (as was first suggested by Bateson and 

 by Castle). Individuals which receive the factor from both parents 

 would be of one sex, those to which it is transmitted by one parent only, 

 of the other sex. But formidable difficulties are involved (1) in the 

 evidence that the egg may influence the sex in cases in which observa- 

 tions on chromosomes indicate that the sex should be determined by 

 the spermatozoon ; and (2) in the evidence that the sex may be occa- 

 sionally modified after fertilization by influences acting on the embryo 

 or even later in life. 



The author is inclined to give up the hypothesis of an unchangeable 

 hereditary entity, the presence of which always causes one sex and its 

 absence the other. He suggests that sex-determination depends on the 

 reciprocal action between an inherited factor and its surroundings. 

 Thus every germ-cell would bear a sex-determining factor, but when 

 this factor has relatively small intensity of action, its effect may be 

 counterbalanced by other causes which alter the physiological relation on 

 which sex-determination depends. Certain extraneous conditions acting 

 on the egg or early embryo may perhaps counteract the effect of the 

 sex chromosome. 



Age of Human Embryo.* — 0. Grosser discusses the relation of 

 ovulation and menstruation in the human female, in its bearing on the 

 problem of determining the age of the young embryo. The literature 

 of the subject is examined and compared, and the author concludes that 

 the time of ovulation varies round a mean, which falls within the first 

 pre-menstrual week. The duration of the tubal migration of the fertilized 

 ovum is not eight to ten days as in many Mammals, but may be more 

 than twenty and is normally at least fourteen. Implantation takes place 

 most frequently in the pre-menstrual period, but is not limited to this. 



Effect of Corpus Luteum Substance on Ovulation in Fowl.f 

 Raymond Pearl and Frank M. Surface find that the ovulation of an 



* Anat. Anzeig., xlvii. (1914) pp. 264-83 (1 fig.). 

 m t Journ. Biol. Chemistry, xix. (1914). pp. 263-78. 



