IV.] THE HEART. 85 



epitheloid lining, is developed out of the splanchnic 

 mesoblast on the ventral side of the throat. But 

 since the first commencements of the heart make 

 their appearance prior to the formation of the throat, 

 the development of this organ is somewhat complicated; 

 and in order to gain a clear conception of the manner 

 in which it takes place the topography of the region 

 where it is formed needs to be very distinctly under- 

 stood. 



In the region where the heart is about to appear, 

 the splanchnopleure is continually being folded in on 

 either side, and these lateral folds are progressively 

 meeting and uniting in the middle line to form the under 

 or ventral wall of the foregut. At any given moment 

 these folds will be found to have completely united in 

 the middle line along a certain distance measured from 

 the point in front where the cleavage of the mesoblast 

 (i.e. the separation into somatopleure and splanch- 

 nopleure) begins, to a particular point farther back. 

 They will here be found to be diverging from the point 

 where they were united, and not only diverging late- 

 rally each from the middle line, but also both turning 

 so as to run in a forward direction to regain the surface 

 of the yolk and rejoin the somatopleure, Fig. 29. In a 

 transverse section taken behind this extreme point of 

 union, or point of divergeace, as we may call it, the 

 splanchnopleure on either side when traced downwards 

 from the axis of the embryo may be seen to bend in 

 towards the middle so as to approach its fellow, and then 

 to run rapidly outwards. Fig. 31, B. A longitudinal 

 section shews that it runs forwards also at the same 

 time, Fig. 29. A section through the very point of 



