;> I ( nmOTA: ox THE SEKO-AMNTOTK' rOXXEPTTOX. 



AVe thiLs see that the prolonGratioii of tlic sero-amniotie connec- 

 tion is st()))[)ed 1iy the closure of the amniotic cavity. Tliis is Ihe 

 ))eri()d of the niaxininni (leve!(^pment of the cpiJilast/'r s('ro-ai]niioiir 

 (uniiit'i-lioti. At tin's time \t has (Fili". 42) in a dorsal view tlie a]>- 

 |>e:iran('(' nf ;i slender streak sliolitly cur\'ed, witli a concavity on tlic 

 riiilit side, and a swollen epihlastic mass at the spot where the am- 

 niotic 1"olds have coalesced (Fio's. 5 and 42). From this time on, 

 aithonprh cells in it may mnlti]>l\^ hy division, it does not seem to 

 enlargv as a whole — ;it any rate, in any oreat degree — but rather 

 grows less and less, being encroached uyion by mesoblastic cells, in a 

 manner to be explained liereafter. until by the tenth dav of the 

 incid^ation il has been entirely I'eplaced bv a xcrDiuhirii Dirsolilasfic 

 coiiiicclioii. 



4. (^hiiiKjcs IN i]ir l\.rtr(i-J'jinhrii()iiii- (lennimil Lai/ns, 



Every one who has stmlied the development of the chick well 

 knows, that from vei'y e;irlv stages the epiblast of the embryo proper is 

 split up into cell-strata. The up]>ermost layer is built of flattened cells, 

 while the »ujder layei* or layei's are composed of slightly columnar cells. 

 The stratification is not confined to the em])ryo j)ro])er, but graduallv 

 extends ceiitrifugally into the epiblast of the amnion. Fig. DO is 

 taken h'oni the dorsal part (fn' its exact locality see the explanation of 

 figures) of a noianal enibrvo of 7.S lioiu's. whose amniotic cavity has 

 just l)een closed ; and in this figure we see that the stratification has 

 advjuiced considerably beyond the lateral augle (A) of the amnion, 

 but that there nre as yet no distinct layers near the sero-amniotic 

 eoîuiection. 



The extra-embryonic piu'ts of the coelomic cavity are at first 

 lined by a single e])ithelial la \-er of mesoblastic cells. From an early 

 |)eriod. howe\er, a I'eticiilnr tissue graduallv spi-eads itself outwards 



