ON THE i'uETAL MEMBIIANES OE OHELONIA. •'> 



|{,;il)1)ir ;iti(l lîats und iiaiacl the two-layered amniotic cap the •' Tro- 

 amiii(jij."' Fleischmann ha.s also found the s«ame state of things in 

 the eat. It seems tlierefoi'e an estabhshed faet that the head-fold of 

 the amnion, when iirst laid, consists throughout the Amniota only 

 of I he epiblast and liypoblast and is therefore of the natiuv of Pi-oam- 

 nion. In liei)tilia, this point is made perha])s more conspicuous hy 

 the sid)se<pienl histoi'y of the fœtal membranes than in other groups. 



It will be seen fr«jm this stage tliat the head ot" the embryo sinks 

 fr(jm the Iirst I)eIow the leNel of the blastoderm. Apart from any 

 ])hylogenetic significance, there is mechanical necessity for its sink- 

 ing in this manner. As soon as the development begins, the white 

 of the egg is rapidly absorbed from the part over the blastoderm 

 which becomes adherent to the innei- surface of the shell membrane. 

 There is therefore no space into whicli the head can grow except 

 towards below. In removing endjryos from the eggs, I availed 

 myself of the llict of tlie ijlastoderm ])ec«;ming adherent to the shell 

 membrajie. for. with a stout pair of scissors, I could easily cut a 

 watch-glass shaped piece of the shell with theshell mendjrane and the 

 embryo adherent io it, and inverting it, I coidd jxjur the preservative 

 iliiid into it, thus using it like a veritable watch glass, (jnly excelling 

 it ill this that it keeps the end)ryo and the Ijlastoderm stretched in 

 their natiu'al positions. 



I am inclined to thijik that the sendlunar ridge at the posterior 

 end (I'ig. 1) i'*^ also caused l)y the posterior heavy end of the embryo 

 sinkini!' int<j the s})ace below . Its section is almost exactly like that 

 of the lateral foM of the [»ermanent amnion (Figs. oO and oOa, PI. V.). 

 Such adventitious ridges seem to be produced here and there witlnnit 

 any regularity (••f. also Fig. 2). They are of a transient nature and 

 take no part in tlie foi'ination of the anmion. 



As later staijes will show, the wliole amniotic sac is 



