THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 633 



At this time the differentiation of head and trunk has already 

 taken place. This is accomplished, first by the establishment of the 

 higher sensory organs in the anterior portion of the body, secondly 

 by the enlargement of the neural tube into the voluminous brain - 

 vesicles, thirdly by the formation of a regular series of visceral c^fts 

 in the walls of the head-gut, which thus also undergo a kind of 

 segmentation (branchiomerism). 



The region of the body which is thus metamorphosed into a head is 

 from the beginning segmented, and is composed, as the Selachians show, 

 of at least nine primitive segments. 



The development of visceral clefts produces still further differences 

 between head and trunk. By the appearance of visceral clefts, the 

 front part of the body-cavity is divided up into several successive head- 

 cavities. By the disappearance of these cavities, parts corresponding 

 to the thoracic and abdominal cavities have become obliterated. 

 Further, there are developed out of the cells composing the walls of 

 the head-cavities important masses of transversely striped muscles for 

 moving and constricting the separate portions of the branchial region 

 of the alimentary canal, whereas in the trunk the voluntary 

 musculature arises exclusively from the primitive segments. In 

 the trunk these masses of muscle spread out both dorsally over the 

 neural tube and also ventrally into the wall of the thorax and 

 abdomen, whereas in the head they remain limited to a small space 

 and do not undergo any extensive development. 



It is only after head and trunk have thus already become in a high 

 degree different that the cartilaginous axial skeleton begins to be formed. 



The latter is therefore a structure of comparatively recent origin, 

 as it also is peculiar to the phylum, Vertebrata, and even here is 

 wanting in the lowest representative, Amphioxus lanceolatus. 



The development of the cartilaginous axial skeleton in the two 

 chief regions of the body is from the beginning partly similar, partly 

 dissimilar. 



The development is similar in so far as the process of chondrifica- 

 tion begins in both head and trunk in the perichordal connective 

 tissue, then extends around the chorda both above and below, 

 ensheathing it, and finally is continued into the connective-tissue 

 layer that envelops the neural tube. 



The dissimilarity is expressed in the occurrence or omission of 

 segmentation. In the trunk under the influence of the musculature 

 there arises a segmentation of the cartilaginous axial skeleton into 

 firm vertebral pieces, alternating with intervertebral ligaments which 



