THE HEART. 463 



the wall of the throat. We may then safely conclude that the 

 double formation of the heart has no morphological significance, 

 and does not, as might at first sight be supposed, imply that the 

 ancestral Vertebrate had two tubes in the place of the present 

 unpaired heart. I have spoken of this point at considerable 

 length, on account of the morphological importance which has 

 been attached to the double formation of the heart. But the 

 views above enunciated are not expressed for the first time. In 

 the Elements of Embryology we say, p. 64, " The 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." Gotte again in his great work 1 appears to maintain 

 similar views, though I do not perfectly understand all his state- 

 ments. In my review of Kolliker's Embryology 2 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 Ver- 

 tebrates, is at first double (PI. n, fig. 6 ao), and, generally 

 speaking, 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 

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

 PI. 11, fig. 8, V. It may be called either the subintestinal or 



1 Entwicklungsgeschichte d. Unke, pp. 779, 780, 781. 

 - Journal of Anatomy and Physiology, Vol. x. p. 794. 



