MESENCHYME 



been traced to an epithelial origin, that is to say, to a folding of 

 the original spherical envelope of the monoblastula, or of one of 

 its derivative folds. 



The Hertwigs in Die Ccelomtheorie, p. 78, emphasise this distinc- 

 tion in the origin of tissues. They point out that in some animal 

 groups a larger proportion of the adult tissues can be traced to 

 foldings of embryonic epithelia than in others. The irregular 

 heap- like groups of cells, which are not spread out as folds of 

 epithelial surface and so often form a large part of the "mesoblast" 

 of animal embryos, they speak of as " mesenchyme." I am inclined 

 to think that the distinction here made is useful. The mesoblast 

 of Ccelomocoela consists of the epithelial fold of the ccelomic 

 pouch (or its representative cells) and of viesenchyme. The question 

 remains as to what is the origin of that mesenchyme. It cannot 



FIG. 16. GASTRULA STAGE 



OF AN ECHINOID SHOWING 

 DEEP ARCHENTERIC INVA- 

 GINATION DEVOID AS YET 



OF COSLOMIC POUCHES, BUT 

 WITH LARGE MESENCHYME 

 CELLS TRAVERSING THK 

 BLASTOCCEL OR CAVITY 

 BETWEEN ECTODERM (epi- 

 blast) AND ENDODERM 

 (hypoblast). (After 8e- 

 leuka, from Korschelt and 

 Heider.) 



be considered as yet sufficiently ascertained to warrant a final 

 conclusion. According to observations made in some groups, 

 mesenchyme is largely derived from epiblast, in others from hypo- 

 blast (Fig. 16), in others its appearance in the blastoccel or space 

 of the primitive embryonic vesicle precedes the formation of archen- 

 teron itself (Fig. 17). I think that we are bound to bring into 

 consideration here the existence in many Ccelentera of a tissue 

 resembling the mesenchyme of Ccelomocoela. In Scyphomedusae, 

 in Ctenophora, and in Anthozoa, branched, fixed, and wandering 

 cells are found in the mesoglcea which seem to be the same thing 

 as a good deal of what is distinguished as "mesenchyme" in Ccelomo- 

 coela. These appear to be derived from both the primitive layers ; 

 some produce spicules, others fibrous substance, others again seem 

 to be amcebocytes with various functions. It appears to be 

 probable that, though it may be necessary to distinguish other 



