OBSERVATIONS ON THE DEVELOPMENT OF RAIA HAMS. 38 



In none of the cases which we have e miined have we found the fcrtus surrounded 

 either by a membrane or by albuminous matter, but in every instance the yelk and 

 the embryo were fully exposed to contact with the water, which entered by the open- 

 ings already described. An albuminous covering may ha\e existed at an earlier period, 

 and have been absorbed. 



Yelk. — After the body of the embryo has become well defined, it is attached to 

 the yelk by a slender umbilical cord about half an inch in length (fig. 2). The 

 yelk has not the pyriform shape so common in other Selachians, but is mail} spheri- 

 cal, though somewhat flattened above and below. The cord has the length just 

 mentioned only temporarily, and soon begins to shorten, and contracts until the 



foetus rests once more upon the surface of the yelk (figs. 6 and 7). The two 



omphalo-mesenteric vessels, common to all vertebrates, carry the blood from the em- 

 bryo to the yelk and back. The artery, a branch of the mesenteric (figs. 2 and 7, a), 

 passes out beneath the head, over the front of the yelk, and descends to the under 

 surface, giving off minute twigs to the right and left ; but the trunk itself does not 

 branch. Dr. John Davy,* in his observations on the development of the torpedo, 

 although he figures a vein surrounding the vascular area in the younger specimens, 

 yet makes no reference to it in the text. Agassiz has observed a similar vessel in 

 the yelk of a dog-fish, and has for the first time pointed out its resemblance to the 

 sinus terminalis of birds. Dr. Davy's figures, taken in connection with those here 

 given, form a complete series. In the youngest of the specimens described by 

 him the sinus is found on the upper surface of the yelk, and quite near the embryo ; 

 in the second, it has receded toward the sides, and the vascular area enlarged to a 

 corresponding degree. In our specimens it is found on the under surface, is of a 

 triangular form (fig. 3), and encloses only a small area. Eventually it contracts still 

 further, and at last wholly disappears, and thus the entire surface of the yelk becomes 

 vascular (figs. 6 and 



As development advances the yelk is gradually withdrawn into the cavity of the 

 abdomen, as in birds ; but the retraction does not appear to be quite complete in 

 the skates until a short time after hatching. In one instance a fully formed skate 

 taken from the egg-case had the yelk reduced to a small flattened mass about two 

 lines in diameter. Very nearly the same condition existed in another, which was 

 already hatched. In a third instance, where the young had been hatched for a longer 

 time, the yelk had been wholly introduced into the cavity of the abdomen ; but a con- 



* 



Researches Anatomical and Physiological, Vol. I. p. 61. 



VOL. IX. 5 



