240 The Alligator and Its Allies 



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 3c the thin entoderm (en) is separated 

 from the much thicker ectoderm (ec) by a consider- 

 able layer of rather loose mesoderm (mes). In 

 Figure ^,71, which is almost exactly median in posi- 

 tion, there is, of course, no mesoderm to be seen 

 in front of the blastopore, and the entoderm 

 shows a considerable increase in thickness, due to 

 the formation of the notochord (nt). The blasto- 

 pore (blp) is the most striking feature of the figure, 

 and is remarkable for its great width in an antero- 

 posterior direction. Its anterior and posterior 

 borders are outlined by sharply defined layers of 

 ectoderm and entoderm. Posterior to the blasto- 

 pore the lower side of the ectoderm is continuous 

 with a considerable mass of cells, the primitive 

 streak {ps). 



STAGE III 



Figures 4, 4a, 5, 5a. and 6-6i (Plates X., XI.) 



"Figures 4 and 4a are of an embryo removed, on 

 June 1 8th, from an egg which had been taken out 

 of an alligator two days before. Figure 4, a dorsal 

 view, is of special interest in that it shows a second- 



