Development of the Fifth Day 265 



manus are three well-formed digits, with a 

 possible fourth, in a very rudimentary condi- 

 tion. The pes has three well-formed digits, 

 and two others that are in a more or less 

 rudimentary condition. 



The ribs and sternum. — The ribs originate 

 as cartilaginous bars in the mesoblast of the 

 body-wall. By the fusion of the ventral ends 

 of certain of these ribs, two longitudinal carti- 

 laginous bars are formed, lying side by side in 

 the ventral body-wall. These bars later seg- 

 ment off from the ribs from which they were 

 formed, and by fusing with each other form a 

 median band of cartilage, the sternum. 



The skull.— It probably will not be practica- 

 ble for the beginner in vertebrate embryology 

 to work out, in the laboratory, the details of the 

 development of the skull ; but the main fea- 

 tures will be given at this time, since it is at 

 (Fig. 72) about this period that the changes to 

 be discussed begin to take place. 



It is convenient to discuss the development 

 of the skull under two heads : (i) the cranium 

 proper ; (2) the skeleton of the visceral arches. 



The cranium proper. — The notochord, 

 which, as has been said, formed a sort of nu- 

 cleus around which the centra of the vertebral 



