308 



THE ELASMOBRANCH FISHES 



processes from the yolk sac of the embryo form close attachment to the uterine 

 wall. Through this attachment nutriment may be secured by the embryo. 



The relation of the uterine wall to the embryo in Mustelus antarcticus (fig. 

 268) shows still another widely different relationship. In this form, according 

 to T. J. Parker (1882), the uterus, by the ingrowth of its lining, becomes 



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Fig. 269. Development of the butterfly ray, PteropJatea micritra. (From Alcock.) 

 A. The embryo in the uterus. B. A tip of a single villus highly magnified. 



divided up into as many rooms as there are embryos within the uterus. The 

 uterine wall is composed of an outer peritoneal lining (p.), a second or thin 

 muscular layer (m.) , and a third or inner mucous lining (mu.) . It is the last- 

 named layer that grows out to form the partitions separating the uterus into 

 rooms. These rooms are filled with fluid in which the embryos lie and by which 

 they are protected. In this type of Elasmobranch we see a device for protecting 

 the developing young which, in a way, is like that in higher animals. In this 

 form, however, the protective sac is produced by the maternal tissue, while 

 in higher forms it is formed by the embryo. 



In Pteroplatea micrura the villi or papillae on the uterine wall of the mother 

 may be numerous and those which are over the spiracle of the embryo become 

 long and strap-shaped (fig. 269a) . In an embryo that is far advanced the yolk 



