494 



TEE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



serous and mucous layer's of Pander), is probably 

 connected with or derived from both of these 

 primitive layers, a view which it will afterward 

 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 belong to either ectoderm or endoderm, 

 speedily undergoes further subdivision and dif- 

 ferentiation in connection with the appearance 

 of the embryonic organs which arise from it, and 

 in this respect contrasts greatly with the sim- 

 plicity of structure which remains in the de- 

 veloped 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-or- 

 gans, 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 mus- 

 cles and connective tissue, the muscular walls of 

 the alimentary canal, the heart and blood-ves- 

 sels, the kidneys, and the reproductive organs, 

 thus forming much the greatest bulk of the body 

 in the higher animals. 1 



There is, however, a peculiarity in the mode 

 of the earliest development of the mesoblast 

 which is of great importance in connection 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 lamina?, an outer or 

 upper and an inner or lower, and the separation 

 of these by an interval or cavity which corre- 

 sponds 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, 



1 If we reserve the words ectoderm and endoderm to 

 designate the two layers of the primary bilaminar blasto- 

 derm, we may apply the terms epiblast and hypoblast to 

 their derivatives after the formation of the mesoderm, and 

 indicate the relations of the whole to the secondary or 

 quadrilaminar blastoderm, by the accompanying table: 



(•Ectoderm | Epiblast 1 



{ Me 8 oderm.JST at, ; p,en . re -- ' p « on '' nr V 



Primary 

 Blastoderm. 



t Endoderm 



\ Splanchuopleure. ) Blastoderm. 

 Hypoblast. 



viewed in the more extended series of animals 

 down to the annuloida, may receive the more gen- 

 eral appellation of pleuro-splanchnic or parieto- 

 visceral cavity, or, shortly, the axiom. Thus, 

 from an early period in the vertebrate embryo, 

 and in a considerable number of the invertebrate, 

 a division of the mesoderm takes place into the 

 somato-pleural or outer lamina, and the splanch- 

 no-pleural or inner lamina ; the outer being the 

 seat of formation of the dermal, muscular, and 

 osseous systems — the volunto-motory of Kemak ; 

 and the inner of the muscular wall of the aliment- 

 ary canal, as well as of the contractile substance 

 of the heart and the vascular system generally. 



It is interesting to find that there is a corre- 

 spondence between the later division of the me- 

 soderm of the higher animals derived from the 

 two primitive blastodermic laminas, and the origi- 

 nal absence of mesodermic structure in the low- 

 est animals, followed by the gradual appearance, 

 first of one layer (the external muscular in the 

 higher ccelenterata), and soon afterward by the 

 two divisions or lamina? with the intermediate 

 ccelom. 



In this account of what may be termed the 

 organized foundation of the new being, I have 

 entered into some detail, because I felt that our 

 conception of any relation subsisting between the 

 ontogenetic history of animals and their phylo- 

 genetic evolution can only be formed from the 

 careful study of the earliest phenomena of em- 

 bryonic organization. But, notwithstanding the 

 many difficulties which unquestionably still block 

 the way, I am inclined to think that there is great 

 probability in the view of a common bilaminar 

 origin for the embryo of all animals above the 

 protozoa, and that the vertebrate equally with 

 the invertebrate animals may be shown to pos- 

 sess in the first stages of their blastodermic or 

 embryonic formation the two primitive layers of 

 ectoderm and endoderm. 



To attempt, however, to pursue the history of 

 the development of animals in detail would be 

 equivalent to inflicting upon you a complete sys- 

 tem of human and comparative anatomy. But 

 I cannot leave the subject abruptly without an 

 endeavor to point out in the briefest possible 

 manner the bearing of one or two of the leading 

 facts in embryology upon the general relation of 

 ontogeny and phylogeny. 



We are here brought into the contemplation 

 of those remarkable changes, all capable of being 

 observed and demonstrated, by which the com- 

 plex organization of the body is gradually built 

 up out of the elementary materials furnished by 



