6 COMPARATIVE ANATOMY 



(Fig. 6, A and B) with its glands, including the thyroid, thymus, 

 liver and pancreas, as well as to the epithelial parts of the gill- 

 sacs and lungs. 



Though we may look upon the epiblast and hypoblast, — that 

 is, both the primary germinal layers— as arising in the manner 

 above described, the problem as to the origin of the mesoUast is as 

 yet by no means settled. All that can be said at present is briefly 

 as follows : — The mesoblast is a secondary formation, and is phylo- 

 genetically younger than the other two germinal layers; both 

 as regards the origin of its cells and histologically, it is of a com- 

 pound nature, and thus forms a marked contrast to the germinal 

 layers proper. Reminding one in many points of the " mesenchyme " 

 of Invertebrates, it always arises at first from the point where 



r-F 



Fig. 5. — Gastrula. 

 Ekt, epiblast ; Ent, hjpoblast ; B/p, blastopore ; V, archenteron. 



epiblast and hypoblast pass into one another, that is, from the 

 region of the blastopore, or, what comes to the same thing in the 

 higher Vertebrates, from the ^jri/^'i^ire streak. Originating from 

 between the other two layers, one of its first and most important 

 functions is the formation of blood-cells ; later it gives rise to the 

 heart, vessels, suppm'ting and connecting svibstances (connective-tissue, 

 adipose tissue, cartilage, and bone), serous membranes (peritoneum, 

 pleura, pericardium, arachnoid), muscles, and almost the entire 

 e:vcrctory and reproductive a^p^daratus. 



A cleft appearing in the mesoblastic tissue divides it into a 

 2Mrietcd or somatic layer (Fig. 6, A and B), lying along the inner 

 side of the epiblast, and into a visceral or splanchnic layer, which 

 becomes attached to the outer side of the hypoblast. The former, 

 together with the epiblast to which it is united, constitutes the 



