26 ART. n. — p. H ATTA. 



roof of the archenteron is no longer formed of the irregular 

 epithelium of macromeric origin {jnac. ep.) alone, but a regular 

 columnar epithelium (i. mic. ep.) is added to form the posterior 

 section of it. The latter part is not only similar in every 

 histological respect to the outer epithelium {Fi(j. 24 e. mic. ep).), 

 but is directly continuous with it at the dorsal margin of the 

 blastoporic aperture. No doubt both have the same micromeric 

 origin. The anterior limit of the inner micromeric epithelium 

 {Fig. 19, *) against the macromeric epithelium is by no means 

 distinct, but both the layers gradually pass over from one to the 

 other. Traced towards the lateral part, the micromeric epithelium 

 forming the archenteric roof gradually decreases in its antero- 

 posterior extent, finally to get lost at the blastoporic margin still 

 represented by the junction line. Therefore, the micromeric 

 roof is at present posteriorly limited by a crescentic outline. 



The contribution of the micromeric layer to the formation 

 of the archenteric roof has already been pointed out by Balfour^^ ; 

 but it was more clearly made known by Goettc'* and Lwoff."'- 

 Goetto's view differs, however, from the accounts given above by 

 me in so far as concern the origin and the extent of the layer. 

 Goette's assumption of the gastrulation, which Lwoff confirms, 

 attributes micromeric origin to the archenteric roof in its whole 

 extent, while according to the results of my present work, the 

 anterior part of the roof is represented by macromeric layer. 

 At any rate, so long as the blastoporic lip extends in the 

 form of a large arc {Figs. 5b, 6c, and 7c), the archenteric 

 roof is formed entirely of the irregular macromeric epithelium 



1) Balfour: Comparative Embryology, vol. IT, p. 85. A.Shipley gives a brief account 

 of the same fact {loc. cit., p. 5). 



2) loc. cit. 



3) loc. cil. 



