624 SKATES' EGGS AND YOUNG. 
Selachians, but is nearly spherical, 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. 91 and 93). The two omphalo-mesenteric 
vessels, common to all vertebrates, carry the blood from the embryo to 
the yelk and back. The artery, a branch of the mesenteric (figs 86 and 93, 
a), passes out beneath the head, over the front of the yelk, and descends 
to the under surface, giv ing off minute twigs to the right and left; but 
the trunk itself does not branch. Dr. John Dav y, in his obsertáilólé on 
the development of the tor pedo, although he figures a vein surrounding 
the vascular area in the younger Specimens, yet makes no reference to it 
in Vi text. Agassiz has observed a similar vessel in the yelk of a dog- 
and has for the first time pointed out its resemblance to the sinus 
Woo of birds. Dr. Davy's figures, taken in connection with those 
here given, form a complete series. Inthe young- 
est 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 
towards the sides, and the vascular area enlarged 
to à corresponding degree. In our specimens it is 
found on the under surface, is of a triangular form 
(fig. 87), and encloses only a small area. Eventu- 
ally it contracts still further, and at last wholly 
disappears, and thus the entire surface of the yelk 
becomes vascular (figs. 91 and 93). 
As development advances the yelk is gradually 
withdrawn into the cavity of the abdomen, as in 
birds; but the retraction does not npe e 
quite complete in the skates until a short time 
after hatching. In one instance a fully done skate 
taken from the egg-case had the yelk reduced to a 
Iu E icm d pte small flattened mass about two lines in diameter. 
Freies MU M owing Very nearly the same condition existed in another, 
ventral fing, 5; ag m which was already hatched. In a third instance, 
where the young had been hatched side a longer 
time, the yelk had been wholly introduced into the cavity o abdo- 
men; but a considerable mass of it, still within the Hedge cavity, 
remained to be absorbed (fig. 96, a); where, as in A newly hatched chick, 
it serves as a reservoir of nourishment. Dr. Davy states that, in the 
torpedo, the young fish is nourished by the yelk for six weeks after birth. 
In all cases we have found the vitelline duct entering the intestine just 
above the spiral valve.” 
orm of the Fetus.—The general form of the youngest specimen is 
long. slender, "€ vimm d tapers to a point backward, as in fig. 86, and 
may be de in one word as eel-shaped. The head presents two 
rounded suine one of them forward (figs. 89 and 90, d), forming the 

