22 ARTHfTR DRNDY. 



from the one to the other is not always by any means so 

 abrupt. Just outside the margin of the area pelluciila blood 

 islands {B. I.) are making their appearance. These seem to me 

 to De derived from the sheet of mesoblast which grows out 

 from the primitive streak, and which extends about as far as 

 the germinal wall. 



Stage E (figs. 15—34). 



I refer to this stage embryos numbered 56 and 64, found on 

 Stephens Island about the end of November, and removed 

 from the eggs on December 9th. The two agree very closely 

 with one another in all essential features, and differ very 

 conspicuously from the preceding stage. Tiiis difference is 

 due chiefly to the development of the pro-amnion and amnion, 

 which has taken place in such a manner that the anterior end 

 of the embryo completely enveloped in its pro-amniotic 

 covering, now lies freely beneath the general surface of the 

 blastoderm, instead of above it as in the preceding stages, while 

 a large crescentic aperture, situated on the surface of the 

 blastoderm in front of the primitive streak, leads into the 

 amniotic cavity behind (vide figs. 15 — 20). 



Although all the stages in the process have not been actually 

 observed, it is evident that the pro-amnion and amnion 

 develop in the following way : — The anterior end of the embryo 

 sinks down, carrying with it the part of the blastoderm (pro- 

 amnion) which lies beneath. The blastoderm (pro-amnion) 

 thus pushed downward seems to become stretched at the same 

 time, and thus forms a very thin sac (figs. 22 — 27, Pro- 

 Am.) whose lips meet together and unite in the mid-dorsal 

 line above the head. The pro-amnion thus formed around 

 the anterior end of the embryo is composed of two layers, 

 which must be regarded as epiblastic and hypoblastic re- 

 spectively, with no distinct mesoblast between them. The 

 cells of both layers have become flattened out, and so closely 

 pressed together that it is difficult, if not impossible, to dis- 

 tingtiish them except where they pass into the epiblast and 

 hypoblast of the embryo itself (figs. 24, 25). The thin pro- 



