DEVELOPMENT OF THE NERVOUS SYSTEM. 



71 



Fig. 38. — A diagram of segmentation in a generalized vertebrate head. In its 

 general features the diagram follows Petromyzon more closely than any other form. 

 So, the number of the gill clefts, their position relative to the somites, the position 

 of the auditory vesicle, and the formation of hypobranchial muscles from myotome 

 10 and following, are taken from Petromyzon. The neuromeres, the nervus 



