I MESENCHYME 67 



The sum of these amoeboid cells, which proceed along the various 

 evolutionary paths above indicated, were, by 0. Hertwig, given the 

 name Mesenchyme — to distinguish them from the mesothelium, or 

 mesoderm in the restricted sense, in which the cells remain in the 

 form of continuous layers or epithelia. 



The original mesenchyme cells arise by emigration from the 

 pre-existing cell layers. Possibly all three layers give rise to 

 mesenchyme cells. It is the mesoderm however which does so most 

 conspicuously. In an Elasmobranch embryo, for example, active 

 budding off of mesenchyme cells is seen over large areas of the 

 somatic mesoderm and similarly from the inner surface of the 

 splanchnic mesoderm. Most active of all is the production of mesen- 

 chyme cells from the splanchnic mesoderm near the lower end of the 

 mesoderm segment, where the proliferating mesenchyme cells may 

 form a conspicuous mass projecting towards the mesial plane and 

 termed the sclerotome. 1 The special consideration of the sclerotome 

 and of the mesenchyme in general will come most conveniently after 

 the other derivatives of the mesoderm (Chaps. IV., V., VI.). 



1 The use of the word sclerotome in this restricted sense has come to be practically 

 universal in embryological literature and is therefore followed in this volume. The 

 word was invented by Goodsir and defined by him, at the British Association meeting 

 in 1856, as meaning a segment of the supporting tissue or framework (whether 

 "fibrous " or cartilaginous or osseous) in a segmented animal. 



LITERATURE 



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Hatschek. Arb. zool. Inst. Univ. Wien, iv, 1881. 



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