IV.] THE HEART. 65 



complete walls of its own, but rather a cavity, closed in below and at the sides 

 by its mesoblantic walls, and roofed over by the bare hypoblastic under-wall of 

 the foregut. Very shortly, however, the side walls close in above, and thus 

 pinch off the heart as a complete and distinct tube, which becomes quite 

 detached along the greater part of its length from the wall of the gut, though it 

 still remains connected with it, both at the venous and arterial ends. 



Klein (Wien. Sitzungsbericht, LXIII. n., iSyr) considers that the heart 

 is formed from the cells of the mesoblast of the splanchnopleure as a body 

 which, at first solid, subsequently becomes hollow by the conversion of its 

 central cells into blood-corpuscles. The layer of cells immediately surrounding 

 the blood-corpuscles forms the epithelial lining, and subsequently becomes 

 connected with that of the great arteries and veins. 



The following view, which our own observations have led us to adopt, 

 agrees with that of Klein in regarding the heart as being at first a solid thicken- 

 ing of the mesoblast of the splanchnopleure ; but its accordance with the earlier 

 Statements of Von Baer is much more complete. 



In order to understand the formation of the heart it must be distinctly borne 

 in mind that in the region where the heart is about to appear, the splanch- 

 nopleure is continually being folded in on either side, and that these lateral 

 folds are progressively meeting and uniting in the middle line to form the 

 under-wall of the foregut (that which in the adult chick will be the anterior 

 wall of some portion of the alimentary canal). (Compare Chap. n. 5.) 

 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 splanchnopleure) begins, to a particular point farther back. 



At this particular point the folds will have met, but not united, Fig. 17, A. 

 Further back still they will have not even met, but will appear simply inclined 

 towards each other, Fig. 17, B. Or, to put it in another way, they will here be 

 found to be diverging from the point where they were united, and not only diverg- 

 ing laterally 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 somato- 

 pleure, Fig. 1 6. In a transverse section taken behind this extreme point 

 of union, or point of divergence, 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. 17, B. A longitudinal section shews that it runs 

 forwards also at the same time, Fig< 16. A section through the very point 

 of divergence shews the two folds meeting in the middle line and then separating 

 again, so as to form something like the letter X, with the upper limbs con- 

 verging, and the lower limbs diverging. In a section taken in front of the 

 point of divergence, Fig. 18, the lower diverging limbs of the X have disappeared 

 altogether; nothing is left but the upper limbs, v\hich, completely united in the 

 middle line, form the under- wall of the foregut. 



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. 



When the heart is about to be formed, thickenings are observed in the 

 mesoblast of the splanchnopleure, along the diverging folds, i.e. along the 

 lower limbs of the X just behind the point of divergence. Ihese thickenings 

 are continued into each other by a similar thickening of the mesoblast ex- 

 tending through the point of divergence itself. 



At first there is no thickening of the mesoblast in front of the point of 

 divergence, i.e. along the under- wall of the foregut. As the point of diverg- 



E. 5 



