224 NIKOLAS KLEINENBERG. 



the mesoderm, an origin from the two large cells, and these, 

 according to Kowalewsky, were originally elements of the 

 endoderm, from which they separated and approached nearer 

 the surface. In this case the large cells would merely be 

 the part which unites the mesoderm with the endoderm ; 

 and the derivation of the first from the second, though not 

 direct, would be none the less a fact. Bat, as is indicated 

 in what precedes, I am unable to agree with the assertions 

 of the Russian embryologist, because in L. trapezoides the 

 mesoblast cells are distinguishable before the arrangement 

 of the embryoplastic material into distinct layers is recog- 

 nisable ; because at first these cells occupy a position on the 

 surface, with a large part projecting freely, and, changing 

 their position, become pushed from without inwards, instead 

 of coming from a deep layer to the surface; and, finally, 

 because in no respect, neither in the quality of their proto- 

 plasm, nor of their nucleus, do they show any resemblance 

 to endoderm cells. After this they should certainly be con- 

 sidered ectodermic elements, if the earliness of their appear- 

 ance, before the definite foundation of the layers, did not 

 render the question almost insoluble. Besides, I am not at 

 all convinced that the affair takes place in L. ruhellus, as 

 Kowalewsky supposes ; the figures which should bear 

 witness to his assertion^ do not persuade me at all, and 

 unless he is supported by less equivocal observations I think 

 that his opinion rests on a very doubtful foundation. 



I shall call the two cords or mesoderm, together with the 

 superposed ectoderm, the '' germinal streaks" (Keimstreifen), 

 and shall use this term to make the topographical descrip- 

 tions simpler. In tracing the true origin of the organs it 

 would not be correct to use it, as each streak is composed of 

 two layers of different value, of which the lower, the meso- 

 derm, has precise limits, while the upper, the portion of 

 ectoderm belonging to the " streak," is continuous with the 

 general covering of the body. In treating of the original 

 derivation of an organ I shall always go back to the primitive 

 layers. 



The germinal streaks, when they have reached the head 

 end, must naturally be closely approximated, since they 

 extend over an oval body. But they do not unite at once, 

 but, ceasing to progress, they widen so as to form two pro- 

 jections, like the heads of nails, at the sides of the mouth. 

 (PI. X, fig. 15 pp). A little later, however, the most 

 anterior cells tend from both sides towards the median 

 dorsal line, and when they reach it fuse with those of the 

 ' Loc. cit., plate vi, figs. 10 aud 12. 



