. The elasmobranch fishes . 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 t
. The elasmobranch fishes . 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
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