IO SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 51 



This condition of the notochord, which is found throughout about 

 one-third of the length of the embryo, would give the impression 

 that the notochord is of a distinctly entodermal origin. 



In figure 3/c there is no sign of the medullary groove, th< »ugh ecto- 

 derm (ec) is still much thickened in the middle line. The section 

 passes, posterior to the notochord, through the anterior edge of the 

 ventral opening of the blastopore (blp). The mesoderm (mes) is 

 here again continuous with the entoderm, around the edge of the 

 blastopore, but is distinct from the ectoderm. 



Figure 3/ represents the third section posterior to the preceding. 

 The blastopore, which passes upward and backward through the 

 blastoderm, is seen as an enclosed slit (blp). It is surrounded by a 

 distinct layer of compactly arranged cells continuous with the thick- 

 ened ectoderm (ec) above, with the thin entoderm (en) below, and 

 laterally with the gradually thinning and scattering mesoderm (mes). 



Figure 3;/; is the next section posterior to the one just described. 

 It passes through the dorsal opening of the blastopore (blp), which 

 appears as a deep, narrow cleft with thick ectodermal borders. The 

 three germ layers are still continuous with each other, though the 

 connection of the entoderm with the other two is slight. The sec- 

 tions posterior to this one will be described in the next stage, where 

 they have essentially the same structure and are better preserved. 



Figures 3« and 30 are sagittal sections of an embryo of about the 

 stage under discussion. In both figures the head-fold is seen as a 

 deep loop of ectoderm and entoderm, while the head-fold of the 

 amnion is seen at a. 



The beginning of the foregut is seen in figure 3;/ (fg), which is 

 the more nearly median of the two sections, figure 30 being a short 

 distance to the side of the middle line. 



In figure 30 the thin entoderm (en) is separated from the much 

 thicker ectoderm (ec) by a considerable layer of rather loose meso- 

 derm (mes). In figure 3;/, which is almost exactly median in posi- 

 tion, there is, of course, no mesoderm to be seen in front of the blas- 

 topore, and the entoderm shows a considerable increase in thickness, 

 due to the formation of the notochord (nt). The blastopore (blp) 

 is the most striking feature of the figure, and is remarkable for its 

 great width in an antero-posterior direction. Its anterior and pos- 

 terior borders are outlined by sharply defined layers of ectoderm and 

 entoderm. Posterior to the blastopore the lower side of the ecto- 

 derm is continuous with a considerable mass of cells, the primitive 

 streak (ps). 



