462 DEVELOPMENT OF ELASMOBRANCH FISHES. 



not commenced, and when therefore it would be impossible for it 

 to be formed as a single tube. 



In Birds almost every investigator since von Baer has de- 

 tected more or less clearly the coalescence of two halves to 

 form the unpaired heart 1 . Most investigators have however 

 believed that there was from the first an unpaired anterior sec- 

 tion of the heart, and that only the posterior part was formed 

 by the coalescence of two lateral halves. Professor Darlste His, 

 and more recently Kolliker, have stated that there is no such 

 unpaired anterior section of the heart. My own recent ob- 

 servations 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 continuous with 

 the aortic arches, which do not arise till a later period' 2 . In 

 the Bird the heart arises just behind the completed 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 closttre of the throat, and in those 

 in which it arises as a single cavity it is formed subsequently 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 wall 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 



1 Vide Elements of Embryology, Foster and Balfour, pp. 64-66. 



2 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 double formation 

 of the heart. 



