110 DE. ELIAS METSCHNIKOFP. 



layer, but from the ectoderm or endoderm (generally, perhaps, 

 from the ectoderm), it is evident that the primitive function of 

 the mesoderm must be nutritive, and its relation to the various 

 tissues and organs purely secondary. Apart from any other 

 argument, I would point to such a creature as Halisarca, where 

 there is no musculature at all, and where the greater part of 

 the mesoderm is entirely given up to the performance of nutri- 

 tive functions. 



When I speak of the phagocytoblast, as a whole, I do so be- 

 cause development shows us how intimately the mesoderm, or, 

 at least, the greater part of it, is connected with the endoderm. 

 Apart from the facts in the development of Sponges, which I 

 have elsewhere described, I will mention a few points in the 

 formation of the amosboid mesoderm cells in Echinoderms, 

 Pilidium, &c., which support this view. But I must state, at 

 the outset, that I do not exclude the ectoderm from all share 

 in the formation of the middle layer. I would rather suppose 

 that, in earlier times, when the ectoderm had not so completely 

 lost its iugestive power, and when the phagocytoblast was still 

 partly derived from it, amoeboid cells were frequently budded 

 off from the ectoderm to join the other devouring cells (phago- 

 cytes) in the body. In this way may be explained the ectoder- 

 mal origin of a part of the mesoblast in the larva of Halisarca. 



We must also bear in mind that other elements, besides phag- 

 ocytes, acquired a secondary connection with the mesoderm, 

 such as reproductive, muscular, and other cells. So that what 

 we call mesoderm is really a heterogeneous mixture of elements 

 acquired from various sources at various times ; and the origin 

 of the whole of these, as a single germinal layer, must be re- 

 garded as a comparatively late event. In this view of the 

 mesoderm I agree with Balfour^ and the brothers Hertwig.- 

 In speaking, however, of the mesoderm as a nutritive cell- 

 complex, I have done so because I regard this as its primitive 

 and most important function. A detailed phylogenetic history 

 of the mesoblast throughout the Metazoa seems at present im- 



' ' Comparative Embryology,' vol. ii, p. 286. 



- " Die Actinieu," ' Jenaische Zeitschrift,' 1879, vol. xiv. 



