88 THE SECOND DAY. [CHAP. 



between the splanchnic mesoblast and the wall of the 

 throat (Fig. 30, ht). During their formation (Fig. 30), 

 a thin layer of mesoblast remains in contact with the 

 hypoblast, but connected with the main mass of the 

 mesoblast of the heart by protoplasmic processes. A 

 second layer next becomes split from the main mass of 

 mesoblast, being still connected with the first layer by the 

 above-mentioned protoplasmic processes. These two 

 layers unite to form a tube which constitutes the epithe- 

 lioid lining of the heart ; the lumen of this tube is the 

 cavity of the heart, and soon loses the protoplasmic 

 trabeculae which at first traverse it. The cavity of the 

 heart may thus be described as being formed by a 

 hollowing out of the splanchnic mesoblast. Some of the 

 central cells of the original thickenings probably become 

 blood-corpuscles. 



The thick outer part of the cords of splanchnic meso- 

 blast which form the heart become the muscular walls 

 and peritoneal covering of this organ. The muscular 

 wall of each division of the heart has at first the form 

 of a half tube widely open on its dorsal aspect, that 

 is towards the hypoblast of the gut (Fig. 30 and 32). 

 After the two halves of the heart have coalesced in the 

 manner already explained, the muscular walls grow in 

 towards the middle line on the dorsal side until they 

 meet each other and coalesce, thus forming a complete 

 tube as shewn diagrammatically in Fig. 31, A. They 

 remain, however, at first continuous with the splanchnic 

 mesoblast surrounding the throat, thus forming a pro- 

 visional mesentery — the mesocardium — attaching the 

 heart to the ventral wall of the throat. The epithelioid 

 tubes formed in the two halves of the heart remain for 



