246 ESSENTIALS OF ZOOLOGY 



connected behind the head by nephrotome necks with the peri- 

 pheral mesoderm, which latter now encloses the splanchnocoel. 

 In other words, the coelom is resolved into myocoels and a 

 splanchnocoel. In the region of the head the mesoderm on 

 each side of the front end of the notochord may show some 

 transient signs of segmentation, but it appears usually as a 

 mass of unsegmented mesoderm. Behind the auditory organ 

 segmentation is more marked. Seven head segments have been 

 described, but the anterior ones suffer regression, joining the 

 unsegmented mesoderm in front. Finally, only two or three 

 are left to form the tongue musculature supplied by the 

 hypoglossal nerve developed in association with them. With 

 this exception the myotomes are added regularly with growth 

 at the posterior end, and their number is frequently stated to 

 give an idea of the age of the embryo. In the head, also, the 

 region in front of the notochord is supplied with mesoderm 

 by mesenchyme derived from the mesoderm. 



These mesodermal rudiments have the fate that we have 

 already observed in other vertebrates. The myotomes yield 

 the skeletal musculature, the dermis, and give off sclerotomes 

 which form the mesenchyme sheaths of the notochord and 

 neural canal. The nephrotomes are from about the third 

 to the fifteenth segment behind the head concerned in the 

 formation of the pronephros and its duct. The pronephros 

 is followed by a mesonephros, and finally by a metanephros. 

 The splanchnocoel, or body cavity, is resolved into the anterior 

 pericardial and the posterior peritoneal cavities. 



The mesoderm supplies likewise an abundant mesenchyme 

 which forms the connective tissues and the blood and the blood- 

 vessels. The circulation of the blood in the bird is directed 

 in the first instance to carrying food from the yolk. The vessels 

 arise in the yolk-sac tfiesoderm and extend into the body as 

 vitelline veins. These approximate behind the gills in the 

 floor of the pharynx to form the heart, and diverge anteriorly 

 to encircle the pharynx, and afterwards the blood is carried 

 to the body by tjvo aortae. From the aortae vitelline arteries 

 arise to connect the system with the vessels of the yolk sac. 

 With the development of the allantois the function of respira- 

 tion is taken over, leaving the food supply to the yolk sac. 



