MESENCHYME AND MESOBLAST 29 



thing like certainty ; and I venture to point out that this further 

 stage of progress, like the earlier which started from the first 

 generalisation of Haeckel of Jena, has been gained by the specula- 

 tion and observation of the English school of morphologists. 



(c) The Ccelom aiid the Mesenchyme. 



The recognition of the coelom as a constant factor of the 

 bodily structure of the higher animal phyla, and of its essential 

 nature as a pair of enterocoelous pouches (or in lower forms as 

 possibly a single pouch, or several such pouches), gives the key 

 for the analysis of that mass of cells lying between the outermost 

 layer of the embryo (epiblast) and the innermost layer (hypoblast) 

 to which in Triploblastic animals, i.e. animals with apparently 

 three embryonic cell-layers, the term " mesoblast " has been applied. 



Clearly one factor of this "mesoblast" is the rudiment (fore- 

 cast, Anlage), of the ccelom, whether appearing as a pouch (Fig. 3), 

 or a solid mass of cells (Fig. 11, ), or as a single pair of cells 

 (Fig. 11, A). There are some reasons for supposing that the whole 

 mesoblast is thus accounted for, and that whatever cells appear 

 in the mesoblast outside and beyond the lining cells of the 

 coelomic pouches are only secondary derivatives of the wall of the 

 coelomic pouches. The development of Amphioxus, for instance, 

 seems to be satisfactorily traced to a folding of a sheet of cells, 

 arranged in a superficies one-cell-deep. Thus the embryonic tissues 

 of Amphioxus have a strictly epithelial character : the cells all 

 bound a surface. By a primitive infolding of the vesicular mono- 

 blastula (or one -cell -layered embryonic vesicle), we obtain the 

 archenteron ; by a second elongated infolding the nerve cord ; 

 by an outgrowth of hollow folds from the archenteron, the primitive 

 coalom is formed ; and by subsequent foldings of the wall of this 

 chamber (as shown by Hatschek, Anat. Anzeiger, August 1888), 

 the myocoal and the splanchnocoel (divisions of the co3lom) are 

 formed. All the main tissues, muscular and skeletal, as well as 

 the tissues arising from the lining cells of the gut and from the 

 epiblast, have an epithelial origin ; there is no accumulation of 

 cells in heap-like masses, no development of branching, irregularly 

 grouped series of cells overlying one another and filling up a space 

 between epithelial layers. 



It may be argued accordingly from Amphioxus that, primarily, 

 the whole mesoblast in all cases is nothing but epithelial foldings 

 of the ccelomic pouches, and that any and all separate cells not 

 lying in the plane of the epithelial surface are merely due to 

 secondary detachment and wandering of a precocious character. 

 It is, however, to be noted that even in Amphioxus the formation 

 of the blood-vessels, large and small, and of the blood has not yet 



