REPRODUCTION 



107 



cells derived from the adjacent mesodermal layers, therefore essentially 

 mesenchymatous in nature. These cells arrange themselves to form 

 the very thin wall of a relatively large vessel. The wall is the endothehal 

 Uning or endocardium of the prospective heart. In some cases, at first 

 two endothehal tubes are formed, lying side by side, later coalescing into 

 one. The thick muscular layer (myocardium) of the wall of the heart is 

 derived from the adjacent hj'pomeric mesoderm, but this muscle, unlike 

 that of blood vessels, is striated. In details of structure it differs from 

 the striated muscle of the body walls. The coelomic space on either side 

 of the developing heart remains as the pericardial cavity. The visceral 

 layer of the hypomere, not wholly consumed in giving rise to the myo- 



FiG. 79. — -Diagrammatic transverse sections of developing heart. In A the descend- 

 ing right and left mesodermal hypomeres have nearly met; mesenchyme cells appear 

 between them. In B the layers have met ventrally forming the ventral mesocardium; 

 the enclosed mesenchyme has formed the endocardium. In C the layers have met 

 dorsally to form a dorsal mesocardium; meanwhile the ventral mesocardium has dis- 

 appeared and the right and left coelomic spaces have become the pericardial cavity. 

 c, coelom; ec, ectoderm; en, endoderm; end, endocardium; m, ventral wall of hypomere; 

 p, pericardial cavity; v, mesenchyme cells. (From Kingsley, Comparative Anatomy 

 of Vertebrates.) 



cardium, forms an outer layer on the wall of the heart, the epicardiimi, 

 which remains in continuity with the somatic layer of the hypomere. The 

 latter constitutes the Hning or pericarditmi of the pericardial cavity. 

 The pericardium and epicardium evidently correspond to the peritoneum 

 of the more posterior region of the coelom. The right and left hypomeric 

 layers, meeting above and below the heart, produce mesentery-like con- 

 nexions (dorsal and ventral mesocardium) between the heart and peri- 

 cardial walls. Later these mesocardia are largely, or usually entirely, 

 obliterated leaving the heart in an undivided pericardial cavity. Later a 

 transverse septvmi (Fig. 80) arises to wall off the pericardial cavity from 

 the more posterior coelomic cavity. 



Concerning the origin of the earliest blood cells or corpuscles in the 

 embryo, there is some uncertainty but it seems probable that they have 

 common origin with the walls of the vessels. 



Head, Neck, Diaphragm, Tail. The development of the head region 

 presents many problems of which the most perplexing is that of the 

 segmentation of the head. This problem is discussed in other chapters of 

 this book. The mesoderm of the head is less definitely organized than 

 that of the trunk. It presents some obscure evidences of segmentation, 

 but it is difficult to ascertain how many segments are present and their 



