MESENCHYME AND BLOOD- VESSELS 31 



elements in it, the mesenchyme of Coelomoccela is largely consti- 

 tuted b}' cells which are the mother cells of the skeleto-trophic 

 group of tissues, and are destined to form connective tissue, blood- 

 vessels, and blood. The relation of the mesenchyme cells (as 

 shown in such cases as those represented in Figs. 9 and 10) to 

 the blastocoel or primary cavity of the blastula seems to favour 

 the notion that the blood-vascular system has originated from the 

 blastoccel in co-oj)eration, so to speak, with mesenchyme cells. 

 Whether, as is most probable, the mesenchyme also gives rise to 

 muscle cells and muscular tissue is a matter requiring close inves- 

 tigation of cell-lineage, and whether the muscular tissue so formed 

 is or is not confined to that of the walls of blood-vessels. In 



Fig. 17.— Sections of Two Stages in the Development of Holothur[a tubulosa to 

 SHOW THE Development of Mesenchyme at a Period when the Archenteric 

 invagination is only commencing. 

 mr, raicropyle ; fl, chorion ; s.c, blastoccel ; W, cell-layer of the iiionoblastula ; ep, 



epiblast ; liy, hypoblast ; ae, archenteric invagination ; and ttis, mesenchyme. (After 



Seleuka, from Balfour.) 



Amphioxus we know that the somatic muscles are formed from 

 epithelial cells of the myocad division of the coelom. Is this a 

 primitive or a secondary arrangement ? If primitive, it is possible 

 that erstwhile epithelial cells of the ccelom migrate from the 

 pouch-wall in some other embryonic histories and form part of the 

 mass called mesenchyme. We cannot get further with the analysis 

 of mesenchyme until the first origin and subsequent history of 

 every constituent cell in a series of typical examples has been 

 determined. Meanwhile it is a distinct progress to cease from 

 speaking of coelom -forecast (Anlage) and mesenchyme as one 

 entity, viz. " mesoblast," defying analysis. There is no constant 

 morphological factor to be recognised by the name " mesoblast," as 

 has indeed been apparent for many years. Mesoblast includes, 



