The Embryology of Chlamydoselachus 621 



ring has contracted until now only about one-fifth the area of the yolk mass is not covered 

 by the arterial system. The ring gives off a multitude of branches or small arteries on its 

 outer side. These communicate by capillaries with the forming venous system as 

 Figure 8 shows. As may be seen on the lower side in Figure 8, all the veins on this side 

 of the ring are gathered to form the great vein entering the yolk stalk under the tail of the 

 fishlet — if it were drawn in its normal position. The small veins formed on the upper 

 side of the arterial ring (Figure 7) empty from both sides into the main vein just before it 

 enters the yolk stalk. 



Vitelline Circulation of the 50-mm. Embryo 



Since this little fish is but 7 mm. longer than the 43'mm. specimen, its yolk-vascular 

 system might be expected to be in about the same stage of development. However, on 

 the dorsal surface (Figure 9, plate I), the artery, which is under the tail of the rotated 

 embryo, shows six small branches before its main trunk passes over the equator of the egg 

 to form the normal bifurcation. The venous system on the upper side of this yolk is far 

 better developed than that of the younger embryo (Figure 7, plate I), the whole hinder 

 surface of the egg being thickly covered with small veins. 



In ventral aspect (Figure 10, plate I), it is seen that the arterial ring is in about the 

 same stage of development as is that of the 43-mm. specimen (Figure 8, plate I). Notable 

 IS the fact that the irregularities of the two rings are almost identical. Here there is the 

 same profusion of small arteries radiating outward from the ring, but not a single one 

 on the inside. 



So far as I can find, the earliest portrayal of the closing arterial ring on the ventral 

 surface of an elasmobranch egg was made by Wyman (1867, Fig. 3, plate 1). On the yolk 

 of a rapidly developing embryo of Raia hatis in the selachian stage, the ring has nearly 

 closed, and the yolk-sac circulatory system is in about the stage of that shown in Dean's 

 Figures 9 and 10, plate I, for the 50-mm. Chlamydoselachus. 



Yolk-sac Circulation of the 175-mm. Fish 



Little can be seen of this on the dorsal side of the egg (Figure 11, plate 1). Venous 

 blood vessels seem to cover this side of the yolk pretty thoroughly. On the fish's right is 

 a large vein which may be the principal one going into the yolk stalk. On the ventral 

 surface (Figure 12, plate I), it is shown that the arterial ring is breaking up. Only parts of 

 the original artery are seen and these for the first time give off branches on the insxde of the 

 ring as well as on the outside. The yolk-sac circulatory system of the 175-mm. embryo has 

 plainly reached a high stage of development, and the growth of the little fish must 

 go forward much more rapidly than ever with the incoming of larger amounts of 

 food materials. 



Vitelline Circulation of the 390-mm. Shark 



Compared with the circulatory system of the 175-mm. shark, this stage is noteworthy 

 for the complete absence of the arterial ring. We see extending out under the head of the 

 fish one long arterial trunk which breaks up into a multitude of branches. Hence it is 



