ASYMMETRICAL DUPLICITY IX THE CHICK. 95 



naturally turns therefore to a consideration of the condition of 

 this organ. 



The two heart-tubes lie side by side beneath the fore-gut, and 

 are widely separated for the greater part of their length, actual 

 fusion having occurred at no point. ISTo doubt this condition 

 results from the shortness of the fore-gut and the correlated great 

 width of the pericardium (Pis. II. & III. fig. 4, E 5, fig. 5, E 6, E 7), 

 since this is clearly in harmony with the view expressed by Miss 

 Parker (11) that the approximation of the heart-fcubes is due to 

 the growth in length and decrease in width of the pericai-dium 

 which, under nonnal circumstances, occurs at this stao^e. 



In this connection the phenomenon of so-called "double heart," 

 which sometimes occurs in otherwise apparently normal embryos, 

 seems of interest. In the few examples of this condition which 

 I have had the opportunity of examining, the " duplicity " of the 

 heai-t, really, of coiu^se, a non-fusion of the two heart-tubes, 

 has been associated with an unusually anterior position of the 

 cardiac fold, a condition which we should expect to be always 

 present if the above view as to the cause of the approximation 

 and fusion of the heart-tubes be true. 



It has been mentioned that in this specimen the right heart- 

 tube is of considerably less diameter than the left, while in the 

 normal chick the reverse is the usual condition; it has been 

 suggested by Miss Parker (11) that this condition is a precocious 

 indication of heart-curvature, the lesser limb becoming the 

 concave border of the heart. With reference to the apparent 

 anomaly of specimen E, it may be recalled that (a) occasion- 

 ally in the chick, flexure of the heart is to the left instead of to 

 the right ; (6) that the condition here is possibly a functional 

 or physiologically induced asymmetry, Avhich seems to be the 

 more likely explanation. 



In this connection it is of interest to note that Wilder (19) 

 has pointed out that a second cause of " deformity," at least as 

 regards bilateral symmetry or equality of components, and one 

 that is specially operative in assisting the secondary deformation 

 of a diplopage, is found in the striving among the parts during 

 growth for the best physiological efficiency. This is particularly 

 notable in the unequal degree of development often seen in the 

 duplicate systems of organs (particularly those functional from 

 an early pei'iod, e.g., the circulatory and digestive systems) 

 possessed by a diplopage. 



This inequality of the heart-tubes may perhaps be regarded, 

 then, as an instance of the capacity of the various organs of the 

 body to undergo morphological changes in response to altered 

 physiological conditions, comparable, say, with such a common 

 pathological condition as the cardiac hypertrophy and dilatation 

 induced by valvular incompetence. The inducing cause would be 

 the primary asymmetry of the head and fore-brain (see Pis. I. 

 and II. figs. 3, 4, E 3 & E 4), owing to the under-development ©f 

 the fore-brain and optic evagination on the right side. 



