The germ-cells. 68ö 



in these very cases nothing is more striking in the many remarkable 

 results of their researches, than the small amount of cell-material, out 

 of which the embryo takes its birth. The rest here goes to form the 

 larva, and probably the germ-cells, though this latter point is not as 

 yet sufficiently worked out. And so also in the Elasmobranchs, a 

 large portion of the cleavage-material being set aside as the germ-cells. 



Coming now to the second point, the formation of an embryo. 

 In the skate it would be a difficult, if not an impossible, task to follow 

 its "cell-lineage" back to the early segmentation. If this could be 

 carried out, the whole of the embryo would undoubtedly be traceable 

 to a single cell, representing one of the original primary germ-cells. 

 Theoretically, the whole of the embryo, except possibly its gut, or a 

 portion of it, is derivable from this cell ~ a cell of a certain cleav- 

 age phase. 



Eisig demonstrates for Capitella, that the theoretical scheme of 

 the cleavage may not be realised in fact. But, even with the modifi- 

 cations added in the course of ages, the main parts of the worm 

 arise within the larva from two special cells, known as the "somato- 

 blasts". What he writes concerning these two cells is of extreme 

 interest, and as an example the following may be cited: "Bevor ich 

 in der Schilderung der Eitheilungen fortfahre, möchte ich speciell der 

 Theilungen zweier Zellen, nämlich der beiden Somatoblasten, gedenken, 

 weil diese weder im Tempo, noch im Modus ihrer Theilungen sich 

 dem für die Eifurchung im Uebrigen geltenden Schema unterordnen, 

 dagegen ganz so wie selbständige Centren, oder wie 

 Eier im Ei verhalten, welche unbekümmert um die 

 übrigen Anlagen ihrem speciellen Ziel zustreben"^). 



The two cells here referred to are those, by whose divisions 

 the main parts of the future worm are laid down. The phrases 

 spaced in the above are of great significance. The two cells by their 

 products strive at the attainment of their own special ends, the 

 formation of the "embryo" or worm, and in this they act as independent 

 centres, or as eggs within the egg, not concerning themselves about 

 the remaining foundations. 



With the difference that at present one is obliged to speak of a 

 group of cells, where Eisig is in the more fortunate position of having 



1) Eisig, Hugo, Zur Entwicklungsgeschichte der Capitelliden, in: 

 Mitth. zool. Stat. Neapel, V. 13, p. 1 — 292, 9 plates, 1898. Passage 

 cited p. 17. For a different interpretation of Eisig's finds see: Biol. 

 Ctrbl., V. 22, p. 354—355. 



45* 



