28 ARTHUR DENDY. 



lower, thicker layer, from tlie lowest cells of which the hypo- 

 hlastic lining of the alimentary canal is differentiated. 



Stage F (figs. 35— 46). 



To this stage I refer embryos 61 and 72, collected about the 

 end of November, and removed from the egg on December 9th 

 and 10th. Though on the whole closely agreeing with the 

 last stage, these embryos differ in certain noteworthy respects, 

 so that it seems desirable to deal with them separately. They 

 illustrate well the great difficulty which I have experienced in 

 classifying the embryos, owing to the fact that events do not 

 always take place in exactly the same order in different indi- 

 viduals. 



The external characters of the embryos themselves agree so 

 closely with those of the preceding stage that it seems un- 

 necessary to do more than refer the reader to figs. 37 — 41. 

 Both embryos, however, differ from those described in the last 

 stage, in the remarkable fact that the amniotic cavity is con- 

 tinued backwards for some distance as a narrow canal (the 

 posterior amniotic canal), which opens to the exterior through 

 a crescentic aperture on the surface of the blastoderm at some 

 distance behind the primitive streak (figs. 37, 39, 40, 41). 

 This tubular prolongation of the amniotic cavity may either lie 

 in a straight line with the long axis of the body (fig. 41), or 

 obliquely to it (fig. 39). It is formed by invagination of a strip 

 of modified epiblast continued backwards from the primitive 

 streak, combined with an uprising of the edges of the strip, 

 which unite and close it in above (figs. 45, 46), The modi- 

 fication of the epiblast in question consists in a loss of defini- 

 tion on the part of its cells, the nuclei of which stain less 

 intensely than those of the normal epiblast of the area pellu- 

 cida. These characters are shown in fig. 46, which represents 

 a portion of a transverse section passing through the posterior 

 amniotic opening, where the amnion is still in process of 

 formation by the uprising of the epiblast on either side of the 

 modified strip. When the posterior amniotic canal has be- 

 come closed in, it lies altogether below the superficial epiblast, 



