THE EMBRYOLOGY OF THE OPOSSUM 191 



The veins just described as emptying into the precava are 

 the left cardiac veins, but in the opossum the largest cardiac 

 vein, the vena cordis magna, has been shown by McClure ( '03) 

 to run in the ventral interventricular furrow until it reaches 

 the auriculoventricular groove, then to pass dorsad between 

 the root of the pulmonary artery and the left auricle and 

 empty into the right atrium as in birds. This is also charac- 

 teristic of Thalacynus as Cunningham (1882) has shown. 

 Such a condition is never found in placental mammals. 



The stomach and the intestines. Heuser ('21) studied the 

 digestive system in stages 34 and 35. He found that the 

 stomach becomes enormously larger immediately after birth 

 due to distension with milk (fig. 60), and that its gross 

 anatomy is like that already described for the adult opossum 

 by Bensley ('02). " Glands however," he says, "are nowhere 

 present, nor can any of the elements be regarded as cells as- 

 sociated with the formation of acid as found in the functioning 

 stomach of higher mammals." The stomach, therefore, is a 

 reservoir at this time, and does not otherwise participate in 

 the digestive process. 



The same author observed that : 



Villi are present in all parts of the small intestine, although 

 they are older and more advanced in the duodenum . . . . 

 In many places between the villi there are patches of epithelial 

 cells which are smaller and more darkly stained than those 

 covering the villi .... The large intestine had changed to 

 a much smaller degree relatively in the transformation from 

 the late embryo to the pouch young. Although the external 

 caliber of the colon has increased considerably in the interval 

 of growth, there is but little advance in the epithelial or meso- 

 blastic differentiation .... The relatively slight dilation of 

 the colon and the histological findings indicate that the residue 

 of the foodstuffs reaching the colon in the opossum pouch 

 young is relatively small; practically all of the milk must be 

 disposed of in the upper portions of the alimentary canal. 



The sensory organs. The eye of the opossum at birth is in a 

 very embryonic condition. The lens vesicle is in process of de- 

 veloping a fiber nucleus ; that is to say, the cells of the medial 



