248 RICHARD ASSHETON. 



This is pretty obvious in cases like those of the guinea-pig 

 and Pteropus, in which the amnion is formed by the hollowing 

 out of a portion of the embryonic epiblast, and not by folding. 



Duval's account of the development of the amnion in the 

 bat almost as clearly proves it to be of true epiblastic origin. 



In the rat also it is clearly the embryonic epiblast which 

 forms the amnion. 



In all these cases the subzonal membrane which takes part 

 in the formation of the chorion is trophoblastic, and the true 

 amnion alone is epiblastic. But how is it in such cases as 

 those of the rabbit, pig, and mole ? Is the amnion in these 

 cases also true epiblast ? 



As regards the rabbit, I have nothing further to say, beyond 

 what I have written above. The amniotic folds arise between the 

 hinder end of the embryo and the ectoplacental region. Where 

 exactly the junction between epiblast and trophoblast lies cannot 

 be determined. In my diagram (L^) I have left it uncertain. 



In the pig, however, I believe I have been able to trace the 

 junction. In an embryo thirteen days old, which exhibits the 

 earliest sign of amnion formation, the boundary between the 

 embryonal area and the trophoblast is just recognisable. 



The embryonal area cells are still slightly lighter in colour, 

 the nuclei are on an average larger, and at many places the 

 remains of the membrana hypoblastica limitans are still 

 visible (figs. 26, 27). 



From this specimen it appears that the amniotic folds arise 

 at about the junction of the embryonal area and the tropho- 

 blast. It gives one the impression that when completed the 

 true amnion will be formed of embryonal area epiblast and the 

 false amnion of trophoblast. 



But, although it is exceedingly difficult to follow, I do not 

 think that this is exactly what happens. I fancy that in a 

 specimen in which the amnion, with the exception of the 

 " amnionnabelstrang,'' is completed I can trace the junction 

 between epiblast and trophoblast within the limits of the 

 true amnion, as shown in my diagram Su^, and figs. 28, 

 29, T, E. 



