ZOOLOGY AND BOTANY, MICROSCOPY, ETC, 427 



Ganoids, and Amphibia tliey are slightly, in the Selachii, Teleostei, 

 and Sauropsida greatly, and in the Mammalia most completely altered 

 by cenogeny. The two primary germinal layers are primitively 

 simple epithelia, and are, therefore, to be regarded as primary tissues ; 

 all other tissues are secondary, are derived from them, and are 

 apothelial. Thus the mesoblast is, in all vertebrates, a secondary 

 product of the primary endoblast, and is always developed from paired 

 " sacculi coelomares" ; while the right and left pouches remain separated 

 from one another in the dorsal middle line by the mesentery, they fuse 

 along the ventral middle line, and so give rise to the coelom ; the 

 parietal mesoblast or somatopleure is re-named the lamina inodermalis, 

 and the visceral or splanchnopleure the lamina inogastralis. There 

 are now four secondary layers, three of which owe their origin to the 

 endoblast. The mesenchym of vertebrates, which is largely used in 

 the formation of blood and connective tissue, may arise from any of 

 the four secondary layers, but does arise chiefly from the mesoderm. 

 The ectoderm is called the methorium parietale, and is stated to con- 

 sist of part only of the ectoblast, that is, of the epidermis and its 

 appendages ; the endoderm is called the methorium viscerale, and is 

 stated to consist of but a small number of endoblasts— the epithelium 

 of the mesodseum and all its glandular appendages, and of two divisions 

 of ectoblasts — the epithelium of the stomodeeum and of the procto- 

 dseum. 



The mesoderm is the general complex of all the other parts, and 

 consists of various tissues which arise directly or indirectly from the 

 two primary germinal layers ; these may, according to their origin 

 and their functions, be arranged in five groups. 



The epithelial tissues of Vertebrates fall into four essentially 

 different groups : A. Esepithelium or chrotal epithelium ; this is the 

 epidermis with its glandular and appendicular products, the epithelium 

 of the stomo- and procto-dseum, the ependyma, the sensory epithelium 

 of the sensory organs (retina, &c.). B. Endepithelium or gastral 

 epithelium ; this is the gastrodermis, or epithelium of the enteron and 

 enteric glands. C. Mesepithelium, or coelom-epithelium ; this is the 

 pleuroperitoneal or pericardiac epithelia, the sexual and renal epithe- 

 lia, D, Desmepithelium ; this is the epithelium of the blood and 

 lymph vessels, of articular cavities and bursse mucosse, of serous sacs, 

 with osteoblasts, odontoblasts, &c. 



The nervous tissue is completely or largely ectoblastic in origin, 

 and may be divided into cells and fibres ; the former are ganglionic or 

 sensory, the latter, meduUated or non-medullated. It is very possible 

 that part of this group (e. g, various sympathetic ganglia and plexuses) 

 arises from endoblastic mesenchym-cells, as is the case in many inver- 

 tebrates ; the motor roots of nerves (like the muscles) may arise from 

 mesoblasts. 



The muscular tissue is either altogether or largely of endoblastic 

 origin, and derived from the mesoblasts; the greatest part of the 

 transversely striated musculature arises from the somatopleure, and of 

 the unstriated from the splanchnopleure ; it is possible, however, 

 that part (e, g. the smooth muscles of the corium and of the blood- 



