48 H. VON ^y. schulte 



(fig. 1). If the retrocardiac plate is narrow relatively to the 

 foregut, the myocardial mantles will be widely separated upon 

 the closure of the gut and the ventral mesocardium will precede 

 the dorsal in its formation. This type obtains in the rabbit, 

 and to it the cat conforms with some peculiarities in detail which 

 are recorded below. If, however, the con\'erse obtains, if the 

 retrocardiac plate is broad relative to the width of the foregut, 

 the formation of the dorsal mesocardium will be accelerated. 

 Of this modification of the process the guinea-pig is an example. 

 This in brief is the analysis of Strahl and Carius. Mollier com- 

 ments justly that is it 'not quite correct to say that the closure 

 of the foregut is the cause of the fusion of the anlages of the 

 heart, because this occurs independently and later, a point which 

 KoUiker had already made.' The possibility of a third type of 

 approximation of the mantles must be admitted, such for ex- 

 ample as obtains in the chick, where the retro- and pre-cardiac 

 plates are of such proportions that ventral and dorsal mesocar- 

 diac are formed at approximately the same time. Further it is 

 not necessary to assume that the retrocardiac plate is of the same 

 breadth in its whole extent, and a difference in this respect might 

 reach such a degree as to produce a mixed type of mesocardial 

 formation. 



In marsupials Parker describes a developmental process re- 

 sembling but not identical with the conditions present in the 

 cat. Like that species and the rabbit there is no ventral meso- 

 cardium. On closure of the foregut the mantles are widely sepa- 

 rated, the intervening remnants of the precardiac plates forming 

 a median plate between them. Associated with this middle 

 cardiac plate are mesenchyme cells, later capillaries. Subse- 

 quently the middle cardiac plate is inconspicuous in marsupials 

 not forming a thicked ridge as in the cat (Cf. Parker, fig. 17, 

 with fig. 12 of this paper), nor are the angiocysts of the region 

 assigned a role in effecting the fusion between the endothelial 

 anlage. These first fuse in the region of the bulb. In embryos 

 of 15-16 somites a constriction between bulb and ventricle is 

 present only on the right side, which as a whole exceeds the left 

 side in size. Caudad the ^'entricle is limited by a constriction, 



