342 HIROTA ; ON THE SERO-AMNIOTIC CONNECTION, 



posterior end of the .sero-uumiutic connection. The extru-enibryonic 

 coeJomic cavities of the tw(j sides ure sejKirated by this delta. In a 

 transverse section throuyfh the delta we see tliat the sero-anmiotic 

 c(3nnection is a very thick bridg'e between the outer and tiie inner 

 epi1)Jastic layers of tlie fold, and that the bridge is laterally constricted 

 from both sides by the niesoblastic slieets, so that the connection is 

 bi-concave (Fig. 60). 



As the posterior edge of the amniotic fold with its epiblastic 

 delta grows constantly backwards, and the extra-em br}(jnic coelomic 

 cavities as (-onstantly encroach on the epiblastic mass of the posterior 

 edge, a sheet of cells (Fig. 90, s. a. c.) connecting the amnion and tlie 

 serons euxelope is alwaws left between the mesoblastic sheets of the 

 rwo sides, and thus the sero-amniotic connection or the ' Amnionnaht' 

 is prolonged backwards, so long as the posterior edge grows back- 

 wards. This goes on nntil the head fold meets the tail fold of the 

 amnion. The delta also urows somewhat larder in absolnte size in the 

 course of the backward progress (Figs. o<S-J:2, (iO, h'2, and (H)), Whil(; 

 these processes are being carried on, the curvature of the embryo takes 

 place (Figs. 1-5) and the sero-amniotic connection also l)ecomes curved 

 at the same time. 



The mesoblastic septum, which has Ijeen formed in front of the 

 sero-amniotic connection, is gradually obHterated, and the extra- 

 emlji-yonic coelomic cavities of the two sides become continuous in 

 iront oï the epiblastic sero-anmiotic connection (Figs. 2-4). 



Finally, in Chelonia tlie epiblastic delta is observed in the 

 [)osterioi' edge in comparatively later stages of the folding, l)ut iu 

 it the delta is not thickened dorso-ventrally so much as in the 

 chick. 



