232 THE HEART. 



Darlste His, and more recently Kolliker, have stated tliat 

 there is no such unpaired anterior section of the heart. My 

 own recent observations confirm their conclusions as to the 

 double formation of the heart, though I find that the heart has 

 from the first a A-shaped form. At the apex of the A the two 

 limbs are only separated by a median partition and are not con- 

 tinuous with the aortic arches, which do not arise till a later 

 period \ In the Bird the heart arises just behind the com- 

 pleted throat, and a double formation of the heart appears in fact 

 in all instances to be most distinctly correlated with the non-closure 

 of the throat, a non-closure which it must be noted would render 

 it impossible for the heart to arise otherwise than as a double 

 cavity. 



In the instances in which the heart arises as a double cavity 

 it is formed before the complete closure of the throaty and in 

 those in which it arises as a single cavity it is formed subse- 

 quently to the complete formation of the throat. There is thus a 

 double coincidence which renders the conclusion almost certain, 

 that the formation of the heart as two cavities is a secondary 

 change which has been brought about by variations in the period 

 of the closing in of the tuall of the throat. 



If the closing in of the throat were deferred and yet the 

 primitive time of formation of the heart retained, it is clear 

 that such a condition as may be observed in Birds and Mammals 

 must occur, and that the two halves of the heart must be 

 formed widely apart, and only eventually united on the folding 

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

 the double formation of the heart has no morphological signifi- 

 cance, 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 



1 Professor Bischoff {loc. cit.) throws doubts upon the double formation 

 of the heart, and supports his views by Dr Foster's and my failure to find any 

 trace of a double formation of the heart in the chick. Professor Bischoff must 

 I think have misunderstood our description, which contains a clear account of 

 the doiTblc formation of the heart. 



