172 



lowed by Merkel's chrom- platinum solution, has enabled me. to trace 

 tbe history of the entoderm, and the precise origin of the nerve- 

 chord, nephridia, salivary glands, larval glands etc. 



Each germ-band, as was made clear in my first paper, and as had 

 already been stated by Metschnikoff"^, is composed of three distinct 

 layers, namely: (1) A n epidermal layer, (2) a layer of four lon- 

 gitudinal rows of cells, and (3) a deeper layer, next to the 

 yolk, composed of larger cells. The precise origin of all these 

 layers was very carefully worked out in my paper, and none of my suc- 

 cessors has called in question the accuracy of those results, so far as 

 this point is concerned. It may therefore be considered a settled fact, 

 that the second and third layers of each germ -band are derived from 

 five cells. But the question as to the interpretation of these two layers 

 is one on which opinions are greatly at variance. Metschnikoff held 

 the second layer to be the basis of the nerve-chord, and the third layer 

 to be mesoblastic. Quite independently of this author's paper, which 

 I had quite overlooked, I came to the same conclusion, and accordingly 

 named the four cells from which the second layer is derived »neuro- 

 blasts«, and the single large cell from which the third layer arises, 

 »mesoblastc Hoffmann, in his second paper, attaches the same 

 meaning to the second layer, while maintaining that the third layer gives 

 rise to both the mesoderm and entoderm. Bergh, while conceding the 

 fact that the nerve-chain has its origin in a part of the second layer, 

 remarks with rather more assurance than the case appears to call for, 

 that my statement on this point is a pure assertion. Nusbaum fol- 

 lows Balfour in regarding both the second and third layers as meso- 

 derm, and derives the nerve- chain from a median thickening of 

 the epidermal layer, similar to what has been described by Sa- 

 le nsky in Brmichiobdella (Biol. Centralblatt, 2. Bd., No. 7). 



As to the origin of the entoderm, we learn nothing definite from 

 Nusbaum. This author regards the eight »neuroblasts« as entoderm 

 cells, and their products as mesoblastic (sexual cells) . Ho ff mann denies 

 the cell nature of my »entoplasts«, and holds that the entoderm arises 

 from the germ-bands. Bergh also rejects my view of the origin of the 

 permanent entoderm, and suspects that it has an early origin from cells 

 at the anterior end of the embryo, similarly as in Nephelis. 



Nusbaum's statements on the origin of the nerve- chord are, as I 

 am able to show, utterly at variance with fact ; and, in this connection, 

 I would also say that this author appears to have a very confused idea 



2 »Beiträge zur Entwicklungsgeschichte einiger niederen Thiere.« Vorläufige 

 Mittheilung im Bull, de l'Acad. imp. des Sei., de St. Pétersbourg, t. XV, 1871 ; and 

 in Mélanges Biologiques, t. VIL p. 671 — 673. 



