DEVELOPMEXT OF ELASMOBRANCH FISHES, 235 



intestinal vessel, and as the direct posterior continuation of the 

 cardinal veins. Embryology proves however that the caudal 

 vein is a true subintestinal vessel ^ and that its connection with 

 the cardinals is entirely secondary. 



The invariably late appearance of the cardinal veins in 

 the embryo and their absence in Amphioxus leads me to 

 regard them as additions to the circulatory system w^hich 

 appeared in the Vertebrata themselves, and were not inherited 

 from their ancestors. It would no doubt be easy to point to 

 vessels in existing Annelids which might be regarded as their 

 equivalent, but to do so would be in my opinion to follow 

 an entirely false mor]Dhological scent. 



The circulation of the yolk-sack. 



The observations recorded on this subject are so far as 1 

 am acquainted with them very imperfect, and in most cases 

 the arteries and veins appear to have been transposed. 



Professor Wyman^ hov/ever, gives a short description of 

 the circulation in Raja Batis, in which he rightly identifies 

 the arteries, though he regards the arterial ring which surrounds 

 the vascular area as equivalent to the venous sinus terminalis 

 of the Bird. 



The general features of the circulation are clearly portrayed 

 in the somewhat diagrammatic figures on Plate viii., in which 

 the arteries are represented red, and the veins blue I 



I shall follow the figures on this plate in my descriptions. 



Fig. 1 represents my earliest stage of the circulation of the 

 yolk-sack. At this stage there is visible a single aortic trunk 

 passing forwards from the embryo and dividing into two 

 branches. No venous trunk could be detected with the simple 

 microscope, but probably venous channels were present in the 

 thickened edge of the blastoderm. 



1 The morphological importance of tliis point is considerable. It proves, 

 for instance, that the haemal arches of the vertebrae in the tail (vide pp. 

 152 and 153) potentially, at any rate, encircle the gut and enclose the body- 

 cavity as completely as ribs which meet in the median ventral line may be 

 said to do anteriorly. 



2 Memoirs of the American Academy of Arts and Sciences, Vol, rs. 



^ I may state that my determinations of the arrangement of the circulation 

 were made by actual observation of the flow of the blood under the microscope. 



16—2 



