DEVELOPMENT OF ELASMOBRANCH FISHES. 233 



exact mode of development (of the heart) appears according to 

 our present knowledge to be very different in different cases ; 

 and it seems probable that the differences are in fact the result 

 of variations in the mode of formation and time of closure 

 of the alimentary canal." Gbtte again in his great work^ ap- 

 pears to maintain similar views, though I do not perfectly 

 understand all his statements. In my review of Kolliker's 

 Embryology^ this point is still more distinctly enunciated in 

 the following passage: *'The primitive wide separation and 

 complete independence of the two halves of the heart is 

 certainly surprising ; but we are inclined, provisionally at least, 

 to regard it as a secondary condition due to the late period 

 at which the closing of the throat takes place in Mammals." 



The general circulation. 



The chief points of interest in connection with the general 

 circulation centre round the venous system. The arterial arches 

 present no peculiarities : the dorsal aorta, as in all other Verte- 

 brates, is at first double (PI, x. fig. 6 ao), and, generally speak- 

 ing, the arrangement of the arteries accords with what is already 

 known in other forms. The evolution of the venous system 

 deserves more attention. 



The cardinal veins are comparatively late developments. 

 There is at first one single primitive vein continuous in front 

 with the heart and underlying the alimentary canal through its 

 prseanal and postanal sections. This vein is shewn in section in 

 PL V. fig. 8, V, It may be called either the subintestinal or 

 splanchnic vein. At the cloaca, where the gut enlarges and 

 comes in contact with the skin, this vein is compelled to bifur- 

 cate (PL XVII. fig. 6 d V. can.), and usually the two branches into 

 which it divides are unequal in size. The two branches meet 

 again behind the cloaca and take their course ventral to the 

 postanal section of the gut and terminate close to the end of 

 the tail, PL xvii. fig. 6 c. v. can. In the tail they form what is 

 usually known as the caudal vein. The venous system of 



1 EntwicklungagescMchte d. Unke, p. 779, 780, 781. 

 '-^ Journal of Anatomy and PhysiologT/, Vol. x. p. 794. 



B. IG 



