Fig. 21-5. Among the fishes with cartilaginous skeletons (sharks and skates), two entirely different ways of caring 

 for the young are observed. The top picture is the partly cut away egg case of the barn door skate (Ro/a sto- 

 bu//form/s), showing the embryo skate lying within. These eggs are laid in marine vegetation where the long 

 tendrils on the four corners of the egg case anchor it during the incubation period. 



The lower picture is of a small shark {Mustelus) embryo removed from the uterus of the mother. The long um- 

 bilical cord is attached to the half empty yolk sac which supplies nourishment for the embryo during its early life. 

 In its later embryonic life there is a "placenta" formed in which nutrients and gases pass between the fetal and 

 maternal circulation. 



