THE EMBRYOLOGY OF THE OPOSSUM 189 



This curious arrangement, however, is only transitional in 

 the opossum, for by the time the pouch young is 2 weeks old all 

 of the nuclei have migrated to the periphery, and the whole 

 fiber is quite like that of any other mammalian striated muscle. 



Testis cords and the urinogenital organs. Though sex 

 cannot, be distinguished externally until some 12 to 14 days 

 after birth, the first histological differentiation can be recog- 

 nized in stage 35. The female gonad at this time still apears 

 indifferent, but the male gonad develops clearly defined testis 

 cords containing the primordial germ cells. 



At this time there is still no sign of a mullerian duct (figs. 

 54 and 59). The ureters as they enter the bladder are medial 

 to the wolflfian ducts. Tubule anlagen are just beginning to 

 form in the metanephroi. The mesonephroi are highly de- 

 veloped and functional. These primitive kidneys remain func- 

 tional for about a week after birth in the opossum. 



The heart and the cardiac veins. A short resume of 

 previous stages will help to clarify the following description. 

 During the concrescence of the heart tubes the primary head 

 vein and the umbilical vein on each side fuse together and 

 then develop a communication with the vitelline vein on each 

 side to form a sinus venosus (stage 25). At this time the 

 paired heart tubes have fused only in the bulbar region. In 

 stage 26 the ventricles fuse, and in stage 27 the atria fuse and 

 the two venous sinuses open into the single atrium separately 

 on either side of the foregut. At stage 30 the two sinsuses 

 have come together so that there is only one valve into the 

 single atrium, and the septum primum now begins to form 

 just to the left of this valve, so that when the single atrium 

 becomes divided into two again, this valve empties into the 

 right one of the two new atria. 



The twisting of the heart has progressed so far at this time 

 that the left precava is pulled up tight against the atrium of 

 the left side, passes beneath the pulmonary vein, and at the 

 back of the heart lies in the crease between the atrium and 

 the ventricle. Here in stage 35 it receives one or more small 

 tributary veins from the wall of the left ventricle before it 



