THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 79 



A 'priori, such a derivation seems highly improbable ; and yet it 

 is precisely the manner in which embryology teaches 11s that the 

 heart and ventral aorta of the vertebrate have arisen. 



The Origin of the Invertebrate Heart and the Origin of the 



Vertebrate Heart. 



Not only does the vertebrate heart differ from that of the inverte- 

 brate, in that it is branchial while the latter is systemic, but also it 

 is unique in its mode of formation in the embryo. In the Appen- 

 diculata the heart is formed as a single organ in the mid-dorsal line 

 by the growth of the two lateral plates of mesoblast dorsalwards, 

 the heart being formed where they meet. In Mammalia and Aves, 

 the heart and ventral aorta commence as a pair of longitudinal veins, 

 one on each side of the commencing notochord. 



If the embryo be removed from the yolk, the surface of the embryo 

 covering these two venous trunks can be spoken of as the ventral 

 surface of the embryo at that stage, and indeed we find that in the 

 present day there is an increasing tendency to speak of this surface 

 as the ventral surface of the embryo. Thus, Mitsukuri, in his studies 

 of chelonian embryos, lays great stress on the importance of surface 

 views and when the embryo has been removed from the yolk, 

 figures and speaks of its ventral surface. So, also, Locy and ISTeal 

 find that the best method of seeing the early segments of the embryo 

 is to remove the embryo from the yolk, and examine what they speak 

 of as a ventral view. At the period, then, before the formation of the 

 throat, we may say that on the ventral surface of the embryo a pair 

 of longitudinal venous sinuses are found, one on each side of the mid- 

 ventral line, which are in the same position with respect to the mid- 

 axis of the embryo as are the longitudinal venous sinuses in Limulus. 



The next step is the formation of the throat by the extension of 

 the layers of the embryo laterally to meet in the mid-line and so 

 form the pharynx, with the consequence that a new ventral surface is 

 formed ; these two veins, as is well known, travel round also, and, 

 meeting together in the new mid-ventral line, form the subintestinal 

 vein, the heart, and the ventral aorta. 



What is true of Mammalia and Aves, has been shown by P. Mayer 

 to be true universally among vertebrates, so that in all cases the heart 

 and ventral aorta have arisen by the coalescence in the new mid- ventral 



