ACTION OF SEX HORMONES IN FOETAL LIFE 391 



fusion of the twin chorions remained. The urinogenital system 

 of the female, described in section 4, was already definitely of 

 the sterile free-martin type. The inference is, therefore, that, 

 fusion had taken place some time previously in order to account 

 for the completeness of the fusion and the transformation of the 

 reproductive system of the female. 



In the case of twin pregnancies in cattle, therefore, the two 

 vesicles starting in opposite horns of the uterus mil meet in the 

 body of the uterus before the 10 mm. stage; the allantoes of the 

 two vesicles will not however meet until about the 15 mm. stage, 

 and the opportunity for vascular anastomosis therefore dates 

 from this time. 



Bonnet ('89) describes a very early t"v\dn pregnancy of the 

 sheep, which confirms in the strongest way my conclusion con- 

 cerning the early time of fusion of twin chorions in ungulates. 

 His description is of so much interest that I quote it entire, 

 he describes a pair of sheep twins 6 mm. long, secured 18 days 

 and 6 hours after copulation, 



deren serosen Hiillen an den sich beriihrenden Enden auf eine Strecke 

 von 6 cm. in einander eingestlilpt imd verklebt, aber noch nieht ver- 

 wachsen waren. Sie Uessen sich vielmehr noch leicht auseinander- 

 losen. Beide Eier maassen zusammen vom freien Ende des einen bis 

 zum freien Ende des anderen 35 cm. und waren in maximo 1.5 cm. weit. 

 Die Ktirze der Eier ist eine imVergleiche zur Lange einzelner Eier in 

 diesem Stadium auffallende; sie betrug bei einem 15, bei dem anderen 

 17 cm. Wahrscheinlich behindern sich die bald einander mit den 

 Spitzen beriihrenden Eier einigermaassen in der sonst normalen 

 Langenentwieklung. 



It will be noted that in this case the ova met at their apices and 

 invaginated one another, and that the stage of such union was 

 only 6 mm. The sheep's uterus is of precisely the same type as 

 the COW" ; fusion follows the union of the ova in the sheep as in the 

 cow; but vascular anastomosis does not occur in the sheep, as I 

 describe in detail later on, and for this reason the female of 

 two-sexed t\\dns remains unaffected in the sheep. 



We have already referred frequently to the vascular anasto- 

 moses between twin foetuses of the cow, and it is now time to 

 describe the matter fully. The working hypothesis with which 



