THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 605 



becomes the foundation of the cranial capsule, and has received the 

 name of membranous primordial cranium. 



Thus far there is an agreement in the development of the 

 vertebral column and of the cranium. With the beginning of 

 the process of chondrification the conditions become more peculiar. 

 Whereas in the region of the spinal cord the skeletogenous layer 

 undergoes a regular differentiation into cartilaginous and connective- 

 tissue parts into vertebrae and vertebral ligaments and is thereby 

 divided into successive movable segments, such a segmentation does 

 not take place in the head. 



Tlie layer of tissue called membranous primordial cranium undergoes 

 continuous chondrification into a non-articulate capsule enveloping the 

 brain-vesicles. If we go through the whole series of Vertebrates 

 down to the lowest, in no one of them is there exhibited a separation 

 into movable segments corresponding to vertebrae. Therefore the 

 anterior part and the remaining part of the axial skeleton pursue from 

 an early period different directions in their development. 



The contrast is intelligible in view of the different duties to be 

 fulfilled in the two regions, and especially in consideration of the 

 different influences which the action of the muscles exercises upon 

 the form of the skeleton. 



In water-inhabiting animals the trunk-musculature is the most 

 important organ of locomotion, for it bends the trunk now in this 

 direction, now in that, and thereby propels it forwards through the 

 water. If, however, the head region were likewise flexible and 

 movable, it would be disadvantageous for forward motion, inasmuch 

 as a rigid part operates as a cut-water. Moreover, the musculature 

 developed on the head assumes a different function, inasmuch as in 

 the grasping of food and in the process of respiration which is 

 accompanied by an enlargement and reduction of the respiratory 

 tract of the alimentary tube it now adducts and then abducts the 

 ventrally situated parts of the axial skeleton. Besides, it is advan- 

 tageous here to have the skeletal axis present firm points of 

 attachment for the muscles. Finally, the voluminous development 

 of the brain and the higher sensory organs is likewise a participating 

 influence tending to make the part of the head that serves for thnir 

 reception an inflexible region. 



In view of these various factors working in the same direction, it 

 becomes intelligible that in the head a segmentation of the axial skeleton 

 is wanting from the beginning. 



In other respects there prevails a great agreement with tho 



