186 H. V. WILSON AND BLACKWELL MARKHAM 



Whether in ordinary development there is a virtual concres- 

 cence of lateral blastopore lips in the midline, and eventual organ- 

 ization of these, is a question which is most definitely answered 

 not by a study of monsters, but by analytical studies on the nor- 

 mal embryo. Some years ago one of the present writers (Wilson, 

 '00, '01) published data which together with the work of 0. 

 Schultze (ref. in Wilson, loc. cit.), Assheton (ref. in Wilson, loc. 

 cit.), Kopsch ('00 and earlier papers), Eycleshymer ('02 and ear- 

 lier papers), Ikeda ('02), seem to make the concrescence theory 

 untenable for the amphibian egg, since they necessitate the con- 

 clusion that a considerable part of the dorsal axial body is formed 

 in situ, viz., in front of the dorsal blastopore lip where it first 

 forms. That the posterior part of the dorsolateral wall of the 

 embryo (gastrula) is produced by the backward growth of the 

 corresponding part of the blastopore lip is admitted by everyone, 

 but it is plainly arbitrary to construe this as a modified form of 

 concrescence. All parts of the blastopore lip grow backward, 

 dorsal, lateral, and ventral. The difference in the distance cov- 

 ered by dorsal and ventral lips in the frog is, to be sure, consider- 

 able, but this inequality is readily understood as a part of the 

 general asymmetry in gastrulation — an asymmetry due to the 

 acquisition and distribution of yolk in the egg, as Balfour long 

 ago pointed out. 



Likewise the data adduced for fishes by Morgan ('95), Kopsch 

 (Kopsch's splendid study published 1904 contains references to 

 his earlier papers), and Sumner ('00, '03) make it practically im- 

 possible to believe that concrescence normally occurs in these 

 forms. 



That the concrescence theory may contain truth as a phylo- 

 genetic theory, in other words, that concrescence may have actu- 

 ally occurred in the evolution of the distinctly bilateral metazoa 

 from coelenterate-like forms, remains of course possible. And 

 with richer and more precise knowledge, such questions doubt- 

 less will be taken up again in the future. 



Lereboullet is often cited as an adherent — in fact, as the first 

 promulgator — of the concrescence theory. But this is to read 

 into his paper ('63) an interpretation that is not, I believe, war- 



