604 EMBRYOLOGY. 



peculiar purposes. For in the morphological plan of Vertebrates the 

 head takes, in comparison with the trunk, a preeminent position ; it 

 is furnished with especially numerous and highly developed organs 

 concentrated into a short space. 



The neural tube has here become differentiated into the volu- 

 minous brain, with its dissimilar regions. In its immediate vicinity 

 have arisen complicated sensory organs such as nose, eye, and ear. 

 Likewise the part of the digestive tube enclosed within the head bears 

 in many ways its peculiar stamp, since it contains the mouth opening 

 and is provided with organs for the reception and trituration of the 

 food, and is pierced by visceral clefts. All of these parts exercise a 

 determining influence on the form of the skeleton, which adapts itself 

 most accurately to the brain, to the sensory organs, and to the 

 functions of the head-gut, and thereby becomes a very complicated 

 apparatus, especially in the higher Vertebrates. 



Embryology sheds a flood of light on the method of the origin 

 of the cephalic skeleton of Vertebrates; it shows the relations to 

 one another of widely different lower and higher forms, and also 

 answers the question, What relation do the vertebral column and 

 head-skeleton sustain to each other in the plan of organisation 

 of Vertebrates ? Consequently the development of the cephalic 

 skeleton proves to be an especially interesting subject, which has 

 always attracted morphologists, and which has incited to careful 

 investigation. 



During the account some comparative-anatomical digressions will 

 be made, which will contribute to the better comprehension of 

 certain facts, especially those treated of in the final section, in 

 which the vertebral theory of the skull will be briefly discussed. 



As in the case of the vertebral column, there are to be distin- 

 guished three stages of development according to the histological 

 character of the sustentative substance : a membranous, a carti- 

 laginous, and a bony. 



The chorda serves as the foundation for the membranous skeleton 

 of the head, and extends forward to the between-brain. At its 

 anterior end there is formed in Amniota the cephalic flexure, by which 

 the axis of the first two brain-vesicles makes an acute angle with 

 the three following ones (fig. 153). Here also the mesenchyme 

 early grows around the chorda and envelops it in a skeletogenous 

 layer, which spreads out from this region laterad and dorsad, 

 enveloping the five brain-vesicles, and is subsequently differentiated 

 into the membranes of the brain and a layer of tissue, which 



