86 THE SECOND DAT. [CHAP. 



divergence shews the two folds meeting in the middle 

 line and then sepai-ating again, so as to form something 

 like the letter x, with the upper limbs converging, and 

 the lower limbs diverging. In a section taken in 

 front of the point of divergence, the lower diverging 

 limbs of the x have disappeared altogether; nothing 

 is left but the upper limbs, which, completely united 

 in the middle line, form the under-wall of the fore- 

 gut. 



As development proceeds, what we have called the 

 point of divergence is continually being carried farther 

 and farther back, so that the distance between it and 

 the point where the somatopleure and splanchnopleure 

 separate from each other in front, i. e. the length of the 

 foregut, is continually increasing. 



In the chick, as we have already stated, the heart 

 commences to be formed in a region where the folds of 

 the splanchnopleure have not yet united to form the 

 ventral wall of the throat, and appears in the form of 

 two thickenings of the mesoblast of the splanchno- 

 pleure, along the diverging folds, i.e. along the lower 

 limbs of the x, just behind the point of divergence. 

 These thickenings are continued into each other by a 

 similar thickening of the mesoblast extending through 

 the point of divergence itself 



The heart has thus at first the form of an inverted 

 V, and consists of two independent cords of splanchnic 

 mesoblast which meet in front, without however uniting. 

 As the folding-in of the splanchnopleure is continued 

 backwards the two diverging halves of the heart are 

 gradually brought together. Thus very soon the develop- 

 ing heart has the form of an inverted Y, consisting of an 



