494 



HISTOLOGY 



down to its base, where their contents are collected and carried away by 

 the vein. 



Just inside the vein, and the connective-tissue layer in which it lies, 

 is the inner circular layer of smooth muscle which is found in all parts 

 of the uterine wall. Outside of this, again, is the longitudinal smooth 

 muscle layer which is covered externally by a peritoneum. 



Oxygen certainly passes from the blood into the uterine fluid, and is 

 taken into the embryonic blood by the embryonic gill filaments described 

 in Chapter XVII. Also, urates are probably returned and excreted 

 through the parental kidney. Carbon dioxide must also be returned. 

 As to whether any food materials are transferred from the parental 



FIG. 463. Myliobatis aquila; outline of a section through the wall of the oviduct in a plane 

 corresponding to the section in last figure. The epithelium on the papillx is respiratory. 

 The thicker cells lining the intermediate pockets are food secreting. (After BRINKMANN.) 



blood to the embryo through the uterine fluid is doubtful. The embryo 

 has a large yolk supply, but when finally born, it weighs much more than 

 the new egg did. The oxygen which it has absorbed might account for 

 part of this extra weight, also the water. On the other hand, the carbon 

 dioxide and urea thrown off would detract from this balance. It thus 

 remains an open question as to whether the parent furnishes its young 

 with food other than oxygen and its original supply of yolk. 



We are so fortunate in this case of doubt as to have evidence on which 

 to base a decision. There are, among the species of elasmobranch fishes, 

 a perfect series of stages, beginning with those that lay an egg, passing 

 through many sharks and rays which hold the young in a uterus to 

 develop on the yolk, as above in the dogfish, and gradually terminating 



