50 ON EMBRYOLOGY AS AN AID TO ANATOMY 



mandibular arch ; the third the hyoidean is in 

 the hyoid arch. Now in the trunk the walls of 

 both the dorsal and ventral divisions of the coelom 

 become converted into muscles, and the cavities of 

 the dorsal division become obliterated owing to the 

 great increase in the thickness of their walls. In 

 the head cavities the walls of both dorsal and 

 ventral divisions become converted into muscles in 

 the same way, and the only difference is that both 

 dorsal and ventral cavities become obliterated 

 instead of the dorsal only. If we bear in mind 

 that the transverse divisions of the dorsal 

 portion of the coelom into segments is obviously 

 part of the general segmentation of the body, 

 and that the similar division of the head cavity 

 is manifestly of the same nature, we see that we 

 may speak of these first three head cavities as 

 indicating three distinct and successive segments 

 of the head comparable to three distinct and 

 successive body segments. 



We have now accomplished by far the most 

 difficult and tedious portion of our task, but let me 

 direct your attention to another set of organs. The 

 central nervous system consists at an early period 

 in all Vertebrates of a tube closed at both ends 

 stretching along the back of the animal from head 

 to tail. This tube is not of uniform calibre, but its 

 anterior part the future brain is from the first 

 wider than the hinder part the future spinal cord. 

 The anterior part is bent in the shape of a hook, 

 and presents along its whole length a series of 

 alternate dilatations and constrictions which are 



