XC REPORT 1877. 



only remark that a view originally taken of this subject by the acute Von 

 Baer appears more and more to gain ground ; and it is this — that the meso- 

 derm, arising as a secondary structure, that is, later than the two primary 

 layers of ectoderm and endoderm (corresponding to the serous and mucous 

 layers of F'ander), is probably connected with or derived from both of these 

 primitive layers, a view which it will afterwards appear is equally important 

 ontogenetically and phylogenetically. 



But whatever may be the first origin of the mesoblast, we know that in 

 the Vertebrata this layer, separating from between the other two, and 

 acquiring rapidly by its cell-multiplication larger proportions and much 

 greater complexity than belongs to either ectoderm or endoderm, speedily 

 undergoes further subdivision and differentiation in connexion with tho 

 appearance of the embryonic organs which arise from it, and in this respect 

 contrasts greatly with the simplicity of structure which remains in the 

 developed parts of the ectodermic and endodermic layers. Thus, while 

 the ectoderm supplies the formative materials for the external covering or 

 epidermis, together with the rudiments of the central nervous organs and 

 principal sense-organs, and the endoderm by itself only gives rise to the 

 epithelial lining of the alimentary canal and the cellular part of the glands 

 connected with it, the mesoblast is the source of far more numerous and 

 complex parts, viz. the whole of the true skin or corium, the vertebral 

 column and osseous system, the external voluntary muscles and connective 

 tissue, the muscular walls of tho alimentary canal, the heart and blood- 

 vessels, the kidneys, and the reproductive organs, thus forming much the 

 greatest bulk of the body in the higher animals. 



There is, however, a peculiarity in the mode of the earliest development 

 of the mesoblast which is of great importance in connexion with the general 

 history of the disposition of parts in the animal body, to which I must now 

 refer. This consists in the division of the mesoblast in all but its central 

 part into two laininpe, an outer or upper and an inner or lower, and the separa- 

 tion of these by an interval or cavity which corresponds to the space existing 

 between the outer wall of our bodies and the deeper viscera, and which, from 

 the point of view of the vertebrate animals is called the pleuro-peritoneal 

 cavity, but, viewed in the more extended series of animals down to the Annu- 

 loida, may receive the more general appellation of pleuro-splanchnic or 

 parieto-visceral cavity, or, shortly, the coelom. Thus, from an early period 

 in the vertebrate embryo, and in a considerable number of tho invertebrate, 

 a division of the mesoderm takes place into the somatopleural or outer 

 lamina and the splanchnopleural or inner lamina — tho outer being the seat 

 of formation of the dermal, muscular, and osseous systems (the volunto- 

 motory of llemak), and the inner of the muscular w T all of the alimentary 

 canal, as well as of the contractile substance of the heart and tho vascular 

 system generally. 



