38 K. MITSUKTJRr. 



fold of the amnion, I think it reasonahle to assume that it was 

 prodiieed by some such meehanical means. En this relati(^n, I think, 

 those inconstant adventitious folds ns o-iven in Fiu's. 1 and 2 (PI. T.) 

 are hio-hly significant. These iind(^ubtedly arise l)y the neio-hhorino- 

 parts sinkino- heh)w. We mit>ht suppose that in tlic earlier stufj-es 

 of development many such folds are produced, different in different 

 emhrvos aeeording to their individunl idinsynerar-ies, and the anterior 

 inid of the amnion may be looked upon merely as one of these. 

 Oidv the canse that ))rodnces it being present in all the embryos 

 :ind acting permanently and augmenting steiidily, finally gnve rise 

 to the structure whieh we call the ninnion, heredity (-)f course helping 

 a great deal. 



The anterior fold of the amnion, when produced, consists only 

 of the hypoblast and epiblast, nnd is called the Troamnicm. AVe now 

 know that this is fonnd in all the groups of the Amniota, and I think 

 we ought to add the stage of the proamnion as of normal occurrence 

 n the development of the amnion. In Chelonia. the dorsal part 

 of the proamnion is for some time entirely epiblastic. Should this 

 be looked upon as a |)rimitive feature or as a secondary one? I am 

 inclined to adopt the former view for two reasons : — 



(1). The inconstant adventitious folds are, as previously stated, 

 always pui-ely e])iblastic and exactly like the lateral folds of the 

 proamnion (Fig. 80a, PI. A'.): hence, it is reas<^nable t(^ conclude that 

 all such folds produced on the surface «^f the blastoderm are ;it first 

 always purelv epiblastic, and the solid epiblastic dorsal sheet of the 

 Proamnion produced by the coalescence of the lateral folds of two 

 sides have reason to be simply epiblastic. 



(2). Tn Olemmys, whose deNt'lopmetit is certaitdy more pruni- 

 tive than that of Tri onyx, the solid dorsal sheet persists for a longer 

 time than in the latter genus, and thei-e is a ccmsiderahle interval of 



