I02 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



cations in other parts of the ccelomic wall must be outlined 

 now. 



As will be seen by the diagram above (Fig. 1 1 o), the middle 

 zone becomes segmented at the same time and in the same way 

 as the myotomes, and when the latter cut loose from the rest of 

 the mesothelial tissue, the contained coelom becomes roofed in 

 above. From the inner or visceral wall of these mesoraeric 

 segments there is now a rapid proliferation of cells upon the 



Fig. III. Diagram of a part of the trunk of an embryonic vertebrate showing 

 the development of the mesothelial tissues, a, aorla; g, neural crest (anlage of 

 spinal ganglion); i, intestine; «, notochord; /, rudiment of pronephric tubule; i, 

 sclerotome; sc, spinal cord; so, somatic layer of mesolhelium; sf, splanchnic layer. 

 (For later conditions compare with Fig. 127.) 



deeper surface, the products of which migrate inward around 

 the notochord, where they eventually give rise to the skeletal 

 structures (vertebrae) surrounding the notochord and central 

 nervous system, from which fact these immigrant cells, divided 

 at first into segments like the coelomic walls which gave them 

 origin, are called sclerotomes. From the method of formation — 

 budding of separate cells instead of an involution of epithelial 

 tissue — these sclerotomes must be regarded as mesenchymatous 

 in nature, and their future fate must be described in connection 

 with that layer. It is only necessary to say here that these 



