56 



EVOLUTION AND GENETICS 



example, a mass of cartilage appears similar to the chondrocranium 

 of the dogfish. This mass later ossifies to form most of the occipi- 

 tal, sphenoid, ethmoid and temporal bones, to which are added 

 the other bones of the skull by development directly from mem- 

 branous regions, without a primary cartilaginous stage (Fig. 37). 



Spinal cord 



Mesodermal segment 



Entoderm 



Coslom 



Fore-gut 

 L. vitelline vein- 

 Splanchnic mesoderm 



Posterior cardinal vein 



Splanchnic mesoderm 

 Liver anlage 



Vitelline vein 



Fig. 34. — Sections of vertebrate embryos, 

 of a chick embryo of two days. 



A, section through the hver anlage 

 {Continued on next page.) 



The vertebrae arise first as a series of hour-glass-shaped struc- 

 tures (Fig. 38), surrounding and finally obHterating the notochord 

 as they take on the definitive form of the adult. The visceral 

 skeleton remains largely cartilaginous, and contributes to the 

 formation of the larynx and upper part of the trachea, the bones 

 of the middle ear, and as in the fishes, the hyoid apparatus and 

 lower jaws. The last is a striking duplication of the state illus- 

 trated by the various fishes. In early embryos sections of the 

 lower jaw show a rod of cartilage on each side. This is Meckel's 

 cartilage which persists as the sole support of the mandible in the 

 sharks. In slightly older embryos a condensation of mesenchymal 



