THE EMBRYOLOGY OF THE OPOSSUM 199 



The miillerian duct tunnnels into the mesonephros and grows 

 caudad as a solid cord which follows the course of the wolffian duct 

 but does not apparently receive any cells therefrom. When it reaches 

 the urinogenital sinus it fuses with its wall at the point which repre- 

 sents the old cloacal horn. The epithelium of this horn meanwhile 

 has proliferated until it has filled the lumen and converted the horn 

 into a solid cord separating both the miillerian duct and the wolffian 

 duct from the cavity of the urinogenital sinus. Of course, the 

 wolffian duct cannot function at all after it loses communication with 

 the cavity of the sinus, and the retrogression of the mesonephros there- 

 after is rapid. The solid sinus cord later becomes the caudal third 

 of the lateral vaginal canal which (as Hartman, '23, has shown) 

 remains uncanalized until just before the first prooestrum. The 

 miillerian duct forms the cranial two-thirds of the vagina. This is 

 the situation which Buchanan and Fraser ('18) found in Trichosurus, 

 and which Hill and Fraser ( '25), on the basis of an histological study 

 of the adult, predicted would be found in Didelphys. 



To revert now to the 1-week-old opossum, in addition to having a 

 functional metanephros and the beginning of a miillerian duct, it also 

 has the beginning of a tectorial membrane in the inner ear, and the 

 cochlear duct has grown out to one and one-half turns (fig. 63). The 

 lagena or apical tip is still actively growing. The vestibular part is just 

 completing the growth which will soon separate it from the sacculus 

 everywhere except at the very slender canalis reuniens. The only part 

 of the duct which has already ceased to grow is the second half of 

 the first turn. This region which ceases to grow first gets a 'head 

 start' in differentiation, and the first appearance of the tectorial 

 membrane is at this point. The differentiation of other structures in 

 the inner ear will follow this same course, that is, each new structure 

 appears first in the apical part of the first coil and later spreads in 

 both directions from this point. 



About 1 weeks after birth sex becomes distinguishable externally 

 on account of the appearance of the pouch rudiments in the female 

 and the scrotal rudiments in the male. Both pouch and scrotum 

 are represented at first by paired bilateral folds. The position of 

 the scrotal folds in the male corresponds to the position of the pos- 

 terior end of the pouch folds in the female. Just as in the female 

 the posterior ends of the marsupial folds approach each other and 

 fuse to form the posterior part of the pouch at a point just cranial 

 to the clitoris; so in the male the scrotal folds approach each other 

 and fuse to form a scrotum just cranial to the penis (fig. 64). In 

 later stages both the pouoh and the scrotum migrate further craniad 

 so that they are finally situated a considerable distance anterior to 

 the clitoris and penis respectively (fig. 5). 



