544 



EMBRYOLOGY. 



rounded by a single layer of cells, which is afterwards developed 

 into the endocardium (end).* Outside of the latter the adjacent 

 cells of the middle germ-layer are thickened ; they furnish the 

 material out of which the cardiac musculature (the myocardium) and 

 the superficial membrane of the heart (pericardium viscerale) arise. 

 The fundament of the heart is attached above [dorsally] to the 

 pharynx (d) and below to the body-wall by the remnant of the 

 mesentery, which persists as a thin membrane. We designate these 

 two parts as the suspensory ligaments of the heart, as back [dorsal] 

 and front [ventral] cardiac mesenteries (hhg, vhg), or as mesocardium 

 posterius and anterius. At this time there is nothing to be seen of 

 a pericardial sac, unless we should designate as such the anterior 



[ventral] region of the body- 

 cavity, from which, as the 

 further course of development 

 will show, the pericardium is 

 chiefly derived. 



In the second type the heart 

 arises from distinct and widely 

 separated halves, as the con- 

 ditions in the Chick and the 

 Rabbit most distinctly teach. 



In the Chick the first traces 

 of the fundament may be de- 

 monstrated at an early period, 

 in embryos with four to six 

 primitive segments. They 

 appear here at a time when 

 the various germ-layers are still spread out flat, at a time when the 

 front part of the embryonic fundament first begins to be elevated as 

 the small cephalic protuberance, and the cephalic portion of the intes- 

 tine is still in the first phases of development. As has already been 

 stated, the intestinal cavity in the Chick is developed by the folding 

 together and fusion of the intestinal plates [splanchnopleure]. If 

 one examines carefully the ridge of an intestinal fold in the very 

 process of being formed (fig. 299 A df), one observes that its visceral 

 middle layer is somewhat thickened, composed of large cells, and 

 separated from the entoblast by a space filled with a jelly-like matrix. 

 In the latter there lie a few isolated cells, which subsequently 



* Relative to the origin of the endothelial sac of the heart, compare the 

 observations given on page 186. ' i; ' lj l '-' 



Fig. 298. Cross section from the same series es 

 that from which fig. 297 was drawn, after 

 RABL. 



d, Epithelium of the intestine ; rnn, visceral, pm, 

 parietal middle layer; hhg, posterior, vhg, 

 anterior mesocardium ; end, endocardium ; 

 h, cavity of the heart ; Ik, ventral part of the 

 bocly-cavity ; ep, epidermis. 



