266 J. THOS. PATTERSON. 



ever, these cells spread out and at the same time become flattened 

 thus filling the intervening spaces and producing the character- 

 istic gut-entoderm. At the stage represented in this figure, the 

 entoderm in its forward growth has not yet reached the anterior 

 limit of the subgerminal cavity, but its free edge ends about .35 

 mm. from this point (Fig. 1 1, L). The anterior part of the cav- 

 ity not yet penetrated by the entoderm is occupied mainly by 

 large yolk masses (Fig. 1 2). In the posterior part of this sec- 

 tion the entoderm is directly continuous with the mass of cells 

 (Fig. 13, D), which in turn is continuous with the inner edge of 

 the germ-wall, and posterior to this wall is the region of over- 

 growth (O). In order to prove the origin and significance of 

 this mass one must have recourse to experimental data. This 

 form of evidence shows that the right and left halves of the dor- 

 sal lip of the blastopore grow toward each other and fuse in the 

 median plane, that is, in the plane of the future longitudinal axis 

 of the embryo. This movement of material from the lateral 

 halves is not confined to the dorsal lip alone, but is participated 

 in by the more lateral portions of the margin, that is, by the 

 horns of the germ-wall. In the large majority of blastoderms, 

 however, the right and left horns of the germ-wall do not turn 

 in along the median line and fuse, but their free ends, upon meeting 

 simply coalesce and grow out over the yolk ' (see Fig. 6). It 

 should also be borne in mind that as the horns are moving to- 

 ward the median line, they are at the time being carried centri- 

 fugally by the expansion of the blastoderm. In this way the 

 fused halves of the blastopore lip are enclosed just anterior to 

 the inner edge of the germ-wall, and hence the mass of cells, re- 

 ferred to above, is derived from the deeper portions of this 

 enclosed lip. This is most apparent immediately after the ends 

 of the horns have met, when the ectoderm is not yet differentiated 

 from the underlying mass. 



The movement of the two lateral halves of the posterior mar- 

 gin toward the median line and their simultaneous fusion must be 

 regarded as a form of " concrescence " — the right and left halves 



1 In the cases in which a "marginal notch" is present, and in the rare cases 

 such as that described by Whitman ('83) the horns of the germ-wall must also turn in 

 and fuse. 



