58 



MESENCHYME 



considered by some as equivalent to trunk -segments. They will be referred to 

 again in a subsequent section. 



IKesenchyme. The term mesenchyme is here employed to denote that part 

 of the middle layer which is the blastema of the connective tissues. Its use 

 involves the recognition of two orders of mesoderm. First, mesoderm in the 

 stricter sense of the term (mesothelium) i.e. that which we have termed the dorsal or 

 segmented mesoderm in earlier sections. It is laid down in a coherent layer, in 

 which the coelom is formed by a process of splitting. It may be considered as 

 typically developed from ccelomic pouches of the archenteron (see p. 46), although 

 in general, owing to the accumulation of yolk in the egg and the secondary 

 modifications resulting therefrom, the rudiment is solid and secondarily excavated. 

 It gives origin to the skeletal muscles, the endothelial lining of the body- cavities, 

 the epithelium of the excretory ducts, the germinal epithelium, and the cortical 

 portion of the suprarenal body. Second, mesenchyme, a syncytial formation 



formed of cells budded 

 off individually from the 

 epithelial layers, or formed 

 by resolution of the meso- 

 derm into a loose mass of 

 anastomosing cells. From 

 it are developed the several 

 . forms of connective tissue, 

 the unstriped muscular 

 tissue, perhaps even striped 

 muscles, and also possibly 

 the blood and blood- 

 vessels, though in respect 

 of the blood there is much 

 difference of opinion. 



The mesenchyme occupies 

 everywhere the intervals be- 

 tween the epithelial layers, 

 and forms a complex on quite 

 a different morphological plane 

 from them. 



Originally introduced by 

 0. and R. Her twig, 1 the term 

 * mesenchyme ' has lost its 

 more strict significance. It is 

 now known to arise from 

 several sites, and though in 



m.p. 



s.p. 



FIG. 83. TRANSVERSE SECTION OP A HUMAN EMBRYO OF THE THIRD 



WEEK TO SHOW DIFFERENTIATION OF MESODERMIC SEGMENT. 



(Kollmann.) 



n.c., neural canal ; ao, aorta ; m.p., muscle-plate ; s.p. t skin-plate ; general a derivative of the 

 sc, sclerotome. mesoderm, it is said to arise in 



some cases directly from the 



entoderm. The circular band of ' mesoblast,' for instance, described (after Hubrecht) in the 

 section on the early formation of the middle layer (footnote, p. 34) as arising from the entoderm 

 in Tarsius and concerned in the formation of the blood-vessels is a case in point. Further, some 

 authors (Kastchenko, Goronowitsch, Sewertzoff, Klaatsch, Julia Platt, Lundborg, Koltzoff) 

 have described tracts of connective tissue arising from the ectoderm. But this has not gone 

 uncontradicted ; and in general it seems proper to hold that the mesenchyme is essentially and 

 primarily of mesodermic origin; although, included in the complex, and indistinguishable in 

 their undifferentiated state from the cells derived from this source, there are possibly other 

 portions of different parentage. Thus among the cells of mesodermic origin many authors 

 have described cells which they believe come from the ectoderm or entoderm. Maurer, for 



Die Colomtheorie, Jena, 1881 ; and Studien z. Bliittertheorie, 188, 1 .. 



