AND TFTK FOETAL ^[EMP.RANES. TX TTTE r'TTTCK. 34^ 



3. Liniil of till' rroIniKialioH of llii' Si'vo-Amtiioiic 

 Convfcfio)!. 



At the (Mid ol' tlio second day or o;u'iy in tlie third day ol' the 

 iiiciih;itioii, ;i ne^v amniotic fold arises hehind the tail end as tlie 

 ])i'iinitive t:ùl I'old (Fiii's. '2 and ()1). The process of its folding is 

 (jiiite similar to that of the first stages (^f the liead fold. Bnt its 

 horse-shoe shaped edge is dh-ected anteriorly and there is never fonnd 

 a nv trace of the mesoblastic septum which has been observed in the 

 head fold. Early in the fourth day, while its cavity is still \'ery 

 shallow, it becomes fused with the head fold at tlie level of the still 

 rudimentary right hind limb (Figs, a and 4). 



The seeming growth of the tail fold in the anterior direction is 

 due, in my opinion, not so much to actual advancement in that 

 direction as to the backward growth of the tail and its consequent 

 sinking into the yolk sac. If the head fold of the amnion arose a 

 little earlier or grew backwards a little more rapidly than it actually 

 does, it would reach the posteri<n* end before any tail fold had risen 

 and the case would be somewhat similar to that of Chelonia. 



When the tail fold approaches the head fold, its anterior epi- 

 blastic edge becomes thickened into an irregular cell-mass, especiallv 

 developed in the median line. Thus the elliptic opening which 

 mai-ks the point of the final closure of the amniotic cavity, or the 

 ' KommunikationsoiFnung ' of Schenk, is surrounded bv a thick 

 epiblastic ring, anteriorly deltoid and posteriorly irregular (Figs. 3. 4. 

 41. and 67). By degrees, as the epiblastic ring becomes thickened, 

 the diameter of tbe opening is reduced until the folds have entirely 

 coalesced, usually in 75-<Sö hours (Figs. 4, 5, and 42). Fig. 89 is 

 a transverse secti(jn througli the o])ening, when it lias just closed, and 

 when the epiblastic laiig is greatly swollen. 



