No. 2.] FORMATION OF THE FISH EMBRYO. 45 1 



It seems to me, therefore, that Rauber's explanation will not 

 hold good, at least for the anterior end of these abnormal fish 

 embryos. The following alternative explanation is suggested. 

 The results may be due to a defect in the formation of the 

 neuro-chordal plate which, instead of concentrating in the axial 

 line, remains flattened out, or thickens at the two sides, pro- 

 ducing, in connection with the mesoderm, the lines of the V. 

 Such an explanation commends itself to me, because, in the 

 first place, it will account for the presence of the layer of cells 

 between the arms of the V; in the second place, because the 

 sections through these embryos, which Rauber has figured, 

 do not show a thickened ectoderm at the limbs of the V, but 

 only a special accumulation of mesoderm; and in the third 

 place, because such abnormalities are directly comparable to 

 those that I have found to form when the sea-water is diluted. 

 I advance these views, not with the intention of setting up 

 an alternative merely formal explanation, but with the hope 

 that these abnormal forms may be reexamined before they are 

 admitted as evidence in favor of a theory of concrescence by 

 apposition. 



In the second category of abnormalities are those cases of 

 double embryos on the same blastoderm. Rauber has shown 

 many cases where two embryos are formed, simultaneously, on 

 the same disc. They may appear near together, or else lie at 

 opposite points of the germ-ring, or at any intermediate points. 

 Rauber believes, and I think his evidence is sufficient, that 

 when these embryos appear near to one another, they gradually 

 approach as they elongate, and we have formed a double-headed 

 embryo with a single body. The proximity of the embryos at 

 the beginning will determine the length of the free portions, 

 since the nearer together they are, so much the sooner will 

 they fuse. 



Rauber holds this to be a demonstration that the body of 

 the fish, as it elongates posteriorly, continually receives addi- 

 tions from the germ-ring to its two sides. 



I see no escape from this conclusion, although, as I shall 

 attempt to show, I do not see that it demonstrates that the 

 posterior elongation is due to apposition. 



