38 SUMMARY OF CUERENT RESEARCHES RELATING TO 



idea reached is that the sterile free-martin is zygotically a femalev 

 modified by the sex-hormones of the male twin, which circulate in both 

 individuals during the foetal life owing to secondary fusion of the- 

 chorions and anastomosis of the foital circulation of the two individuals. 



The author argues that the free-martin is zygotically a female. 

 1. The only basis on which it could be logically interpreted as male is- 

 that it is co-zygotic with its male mate, because it is impossible to 

 suppose that the association of two males in utero should cause the 

 transformation of one of them into a free-martin in a certain definite- 

 proportion of cases. But the free-martin and its male mate arise from 

 separate zygotes. From this point of view the free-martin must be 

 interpreted as zygotically female. 2. The somatic resemblances betweea 

 the free-martin and its mate are not of the order of identical twins, 

 3. The assumption that the free-martin is male leads to an absolutely 

 incomprehensible sex-ratio, while the interpretation that it is female 

 comes nearer fulfilling the expected sex-ratio. From this point of view 

 also the free-martin is female. The only argument for its male nature 

 rests on the internal organs of reproduction, which are more or less of 

 the male type. But the external genitals and the mammary gland are- 

 almost invariably of the female type. 



Lillie discusses a large number of cases and gives figures of great 

 excellence. The facts suggest the theory that the course of embryonic 

 differentiation is largely determined by sex-hormones, circulating in the 

 blood. This secondary differentiation must be distinguished from the 

 primary zygotic determination of the male and female sex. The 

 question why the sex-hormones' of the mother do not affect the 

 reproductive system of male offspring shows the need of further 

 investigation. It is unlikely that there is any cessation of production, 

 of sex-hormones during foetal life; it may be that they are neutralized in 

 some way ; more probably, the placenta is impervious to them. 



The intersexual condition of the free-martin is, on Lillie's theory^ 

 comparable to the intersexual condition in some Gypsy Moths. It is- 

 due, however, to an acceleration or intensification of the male factors of 

 the female zygote by the male hormones. There are many grades of 

 transformation, the ovary approximating towards a testis. Thus, sex in 

 mammals cannot be diagnosed by the character of the gonads alone,, 

 because a testis-bearing individual may develop from a female zygote. 

 The unexpected result is reached that the external genitalia and the 

 mammary gland are more reliable criteria of the female sex than the 

 internal parts. 



Germ-cells of Loggerhead Turtle."— H. E. Jordan has studied 

 twenty-five embryos of the Loggerhead Turtle {Caretta careita) from the 

 second day (five somites, 2 mm. in length) to the thirty-second day of 

 incubation. He found that the early history of the primordial germ- 

 cells is very similar to that described by Allen for the turtle Chrysemys: 

 and by Woods for the dog-fish. The primordial germ-cells migrate 

 during the second day from the yolk-sac endoderm into the lateral 

 border of the area pellucida on each side of the embryonic disk. By 



* Carnegie Inst. Washington, Publication No. 251 (1917) pp. 313-44 (6 pis.). 



