816 



SCIENCE. 



[Vol.. U., No. 47. 



but double, ill its origin. Tlie ectoderm, ento- 

 derm, and purt of tLie mesoderm, he distin- 

 guislied under llie common nanieof ' aicliiblast,' 

 from that portion of the mesoderm which is 

 related to the connective-tissue group (connec- 

 tive tissue proper and endothelia), and which 

 he supposed to grow from the 3'olk (in the chick) 

 into the archiblastic tissue or cells, -which, 

 from the first, are constituent elements of the 

 embryo. Ills maintained that the parablast- 

 cells were derived fiom the white elements of 

 the jolk, but in that respect he is believed to 

 be in error ; nevertheless to His belongs the 

 great honor of having first insisted upon the 

 duplex development of the middle geim-lajer. 

 This knowledge is the key to the solution of 

 one of the fundamental problems of animal 

 morphology. 



The researches of Professor His have been 

 confined to vertebrates. One cannot but feel 

 that his views would have been modilied in 

 many details, if he had included the lower types, 

 also, in his investigations. The discoveries of 

 others, however, have gradually made it clear 

 that among invertebrates, also, the twofold 

 com])osition of the mesoderm exists. The path 

 to this generalization may be said to have 

 opened out upon the announcement bj- Alex- 

 ander Agassiz that in echinoderms the lining 

 of the body-cavity and water-vascular sj^stera 

 is derived from the entoderm. Selenka and 

 others have since shown that the rest of the 

 mesoderm is derived from scattered and isolated 

 cells, which are thrown oflTfrom the other layers 

 into the space between the ectoderm and ento- 

 derm. It was thus clearly shown that in this 

 class of animals the mesoderm primitively 

 consists of two epithelial evaginations and of 

 scattered and independent cells of amoeboid 

 character. The fundamental importance and 

 the far-reaching signiticance of this discover3' 

 were unfortunately not appreciated at the time. 



For several years past I have been accumu- 

 lating materials for a work on ' Comparative 

 histology,' and have meanwhile directed my at- 

 tention chiefly to the classilication and genesis 

 of tissues. These preliminarj- studies led me 

 to various conclusions, among which was the 

 conviction that amoeboid cells wei-e the primi- 

 tive representatives of the mesoderm, and that 

 from them was derived a large [lart of the 

 mesodermic tissues. This view I published in 

 1879 ; ' but the article has, so far as I am aware, 

 been entirely overlooked by subsequent writers, 

 and I thei'efore venture to call especial atten- 

 tion to it now, as the opinion I then advocated 



1 Minot: Preliminary noUce of certain laws of histological 

 differentiation. J'roc. Boston aoc. nat. Mat. xx. 207. 



has since become a current embryological gen- 

 eralization. To the cells I gave the name of 

 ' mesamoeboids.' 



The investigations of Hatschek, whose bril- 

 liant discoveries have not yet received their 

 deserved recognition, have revealed that in 

 Bryozoa, MoUusca, Annelida, and Amphioxus, 

 the mesoderm arises, 1, from cells, such as 

 we have seen maj- be classed under the head 

 of mesamoeboids ; 2, from two paired masses 

 of cells, his ' mesodermstreifen,' whose origin 

 from the entoderm is rendered probable in all 

 cases, and certain in some, by known charac- 

 teristics. These stripes either have from the 

 first, or soon acquire, a distinctly epithelial 

 structure. Hatschek appears to have recog- 

 nized the bearing of his observations nearly as 

 we conceive it now ; and to him, I think, we 

 should accord the honor of having first clearly 

 and definitely recognized the dual histogenesis 

 of the mesoderm. 



F. M. Balfour, in his writings, particularly 

 in his ' Treatise on comparative embrvolog}',' 

 made the next important step by pointing out 

 that the vertebrate mesoderm probably arose as 

 a pair of diverticula from the gastrula cavitj' ; 

 and he gave a new meaning to, and justification 

 of, this theory, by insisting upon the homology* 

 between the blastopore of the Ichthyopsida and 

 the primitive streak of the Amniota ; for from 

 the walls of the former, as well as from the sub- 

 stance of the latter, the paired outgrowths of 

 the middle layer arise. The deficiency in Bal- 

 four's presentation of the subject lies in his 

 failure to recognize the importance of the mes- 

 amoeboids. 



The brothers Ilertwig have published a series 

 of contributions to the solution of the problem, 

 and have embodied their general results in an 

 article entitled the ' Coelomtheorie.' As we 

 have shown, their predecessors had prettj' well 

 established the necessity of regarding the meso- 

 derm as consisting of two parts, — first., the 

 paired epithelial portion derived from the en- 

 toderm, forming the lining of the body-cavity, 

 and giving origin to the peritoneum, muscle- 

 plates, genital glands, etc. ; secondly, scattered 

 cells, giving origin to the connective tissue, 

 the endothelia, vessels of the circulation, the 

 blood, and lymph. These conclusions, how- 

 ever, had never been systematically collated 

 and coherently presented. The brothers Hert- 

 wig performed this task with characteristic 

 abilitv and success. Guided by their own im- 

 portant original researches on several animal 

 tjpes, and utilizing the results of others, they 

 succeeded in demonstrating tbe prevalence of 

 the same composition of the mesoderm in the 



