THE CIRCULATION OF THE YOLK-SACK. 467 



a still more marked feature in a blastoderm of a succeeding 

 stage (fig. 4). 



The circulation in the succeeding stage (fig. 4) (projected in 

 my figure) only differs in details from that of the previous stage. 

 The arterial ring has become much larger, and the portion of 

 the yolk not embraced (x) by it is quite small. Instead of all 

 the branches from the ring being of nearly equal size, two of 

 them are especially developed. The venous system has under- 

 gone no important changes. 



In fig. 5 the circulation is represented at a still later stage. 

 The arterial ring has come to embrace the whole yolk, and as 

 a result of this, has in its turn vanished as did the venous ring 

 before it. At this stage of the circulation there is present a 

 single arterial and a single venous trunk. The arterial trunk is 

 a branch of the dorsal aorta, and the venous trunk originally 

 falls into the heart together with the subintestinal or splanchnic 

 vein, but on the formation of the liver enters this and breaks up 

 into capillaries in it. The venous trunk leaves the body on the 

 right side, and the arterial on the left. 



The most interesting point to be noticed in connection with 

 the yolk-sack circulation of Scyllium is the fact of its being formed 

 on a completely different type to that of the Amniotic Verte- 

 brates. 



THE VASCULAR GLANDS. 



There are in Scyllium two structures which have gone under 

 the name of the suprarenal body. The one of these is an 

 unpaired rod-like body lying between the dorsal aorta and the 

 caudal vein in the region of the posterior end of the kidneys. 

 This body I propose to call the interrenal body. The other is 

 formed by a series of paired bodies situated dorsal to the cardinal 

 veins on branches of the aorta, and arranged segmentally. These 

 bodies I shall call the suprarenal bodies. I propose treating the 

 literature of these bodies together, since they have usually been 

 dealt with in this way, and indeed regarded as parts of the same 

 system. As I hope to shew in the sequel, the origin of these 

 bodies is very different. The interrenal body appears to be 



