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SCIENCE 



[N. S. Vol. Ul. Na 1350 



in development, consisting of vaginal canals, 

 uteri, Fallopian tubes, and small round bodies 

 in the situation of the ovaries. These bodies 

 were sectioned and found to contain within 

 a thin albuginea a mass of closely packed 

 tubules of uniform size, devoid of all indi- 

 cation of sex cells, male or female. 



This specimen is presented in further evi- 

 dence of the hormone theory of sex differ- 

 entiation, in the light of which it is inter- 

 preted. The explanation here offered was 

 suggested by Lillie's beautiful demonstration 

 of the cause of the free-martin in cattle,^ 

 wherein he showed beyond all doubt that the 

 free-martin is a female sterilized by sex 

 hormones from the male co-twin in all cases, 

 and only in such cases, in which the fetal 

 circulations anastomose and mutual blood 

 transfusion occurs. One type of pseudo- 

 hermaphroditism is thus adequately and sim- 

 ply explained and Steinbach's^ assumption of 

 an embryonic gonad containing both male 

 and female interstitial cells as the origin of 

 the antagonistic sex hormones in fetal life 

 becomes superfluous. 



The opossiun briefly described above is 

 interpreted as a reciprocal free-martin, a term 

 I wish to employ for a ses-intergrade which 

 is zygotically male but which in its ontogeny 

 develops female characters. I venture to 

 suggest that it has arisen by " inhibition 

 and stimulation of normal embryonic rudi- 

 ments "^ through the influence of female 

 hormones from a female co-twin. It is be- 

 lieved, therefore, that the case constitutes an 

 answer to the following significant statement 

 by Lillie: 



On the male side there is complete absence of 

 information as to the effects of early embryonic 

 castration and the possible effect of the pres- 

 ence of female hormones in the absence of male 

 hormones.* 



The writer acknowledges, of course, that he 



iF. R. Ldllie, Jour. Exp. Zool, 23, 1917, 391- 

 452. 



2E. Steinach, Pfluger's Arehiv, 144, 1912, 71- 

 108 (see especially page 86). 



8 Lillie, I. c, page 419. 



* Lillie, I. c, page 415. 



ia arguing from analogy; and he would him- 

 self interpose the fiui;her objection that such 

 sex-intergrades are very rare, at least in 

 adults of the opossum. And yet, when one 

 opens a pregnant opossum uterus and ob- 

 serves that it is crowded to capacity with 

 vesicles tmder great pressure and that the 

 chorions are in mutual contact by large siu"- 

 faces, he must wonder why anastomoses of 

 the chorionic circulations do not more fre- 

 quently occur. In this extreme crowding the 

 marsupials are unique. It has occurred to 

 the writer as highly probable that the shell 

 membrane of marsupial eggs, far from being 

 a useless vestige of Saviropsidan ancestry, 

 serves to separate the vesicles until it is safe 

 for them to come into immediate contact, if 

 indeed they ever do. This point will be the 

 subject of further investigation, as also the 

 mechanism by which the male embryo is pro- 

 tected against the hormones of its mother 

 and of its sister embryos. There being no 

 placenta in the opossum, this organ is ruled 

 out in this case." 



There is one condition, however, imder 

 which in the opossum fusion of the chorions 

 is likely to occur, namely in twin eggs where 

 two ova are included in the same egg en- 

 velopes. I have several such eggs in different 

 stages of development.* A comparable case 

 has been described for the rat where fusion 

 took place between the trophoblasts of two 

 eggs that became included in the same 

 decidua.' Professor H. M. Evans, of the Uni- 

 versity of California, told the writer that 

 among several hundred dog embryos of his 

 collection he fotmd one pair of very young 

 twins within the same uterine swelling. The 

 placentse were fused (v.i.). These points 

 raise interesting questions as to the mechan- 

 ism by which the ova are usually distributed 

 singly in oviduct and uterus. 



The sex-intergrade of the opossum here pre- 



5 Cf. Lillie, I. c, footnote, page 415. 



8 See Carl G. Hartman, Jour, of Morph., 32, 

 1919. One twin egg may be seem in each of the 

 batches shown in Fig. 4, PI. 9 and Fig. 2, PI. 1. 



7 V. Widakowioh, ZentralU. f. Physiol., 24, 

 1910, 305. 



