252 ESSENTIALS OF ZOOLOGY 



which form a secondary amnion have the coelom carried over 

 the embryo in the formation of the amnion. 



The mesoderm of the embryo is resolved into myotomes 

 and peripheral mesoderm, the latter enclosing the body cavity. 

 The mesoderm of the head on each side of the notochord forms 

 a mesenchyme except posteriorly, where two or three segments 

 are preserved to form the hypoglossal muscles. The myotomes 

 of the trunk furnish the muscles of the body wall and of the 

 limbs ; they also provide the dermis, and from their inner lower 

 border the sclerotomes which yield the mesenchyme sheaths 

 of the notochord and nerve cord. 



The myotomes are connected at first by nephrotomes with 

 the splanchnocoel mesoderm. Pronephros, mesonephros, and 

 metanephros are formed in succession, and the history of these 

 will be considered in the next chapter. 



The splanchnocoel is resolved in front into the pericardial 

 cavity, and after its separation the lungs extend backwards 

 into dorsal pouches of the pleuroperitoneal cavity. They are 

 thus covered by a visceral layer and surrounded by the 

 parietal layer of the pleura, into the cavity of which they 

 project. They become separated from the peritoneal cavity 

 by the diaphragm, which grows downwards as a fold of 

 mesoderm behind the lungs and the heart. Head cavities do 

 not appear to be formed as such in placental mammals, but 

 they have been found in certain marsupials, and the pre- 

 mandibular cavity yields the oculomotor muscles. 1 



The mesoderm of the yolk sac, mainly that evidently derived 

 from the primitive streak, is the seat of the formation of the 

 first blood vessels and cells. The network is carried inwards 

 towards the body, on each side of which it forms a vessel. 

 With the raising of the body these two vessels are brought 

 into contact to form the heart, and they are enclosed by the 

 pericardium. Diverging in front of the heart the two primi- 

 tive vessels pass round the pharynx to form the dorsal aortae, 

 from which again vitelline arteries are directed to the yolk 

 sac. The primitive circulation is thus that of the large-yolked 

 eggs of birds and reptiles. The serosa is able to take up an 

 exudate from the uterine wall, and the yolk sac is distended 



1 1915, Fraser, Proc. Zool Soc. 



