4*70 EMBRYOLOGY OF THE LOWER VERTEBRATES . 11. 



covered in by a double roof formed by the amnion and the serous 

 membrane. 



The amuiotic fold does not develop with equal activity through- 

 out its extent. Its growth is much mor^ active anteriorly than else- 

 where, with the result that the headward portion of the fold becomes* 

 extended rapidly backwards as an amniotic hood over the head and 

 anterior end of the body of the embryo (cf. Figs. 233, 235, 236). The 

 last remnant of the amniotic opening is consequently situated quite 

 near the hind end of the body. 



Correlated with the later appearance of the amniotic hood at 

 a time when the coelomic cavities are extensively developed it is 

 at no period composed throughout, from side to side, of a simple 

 layer of unsplit ectoderm as was the case with the Chelonian. It 

 is of interest to notice however that the sero-amniotic isthmus has 

 not altogether disappeared, although it never has the breadth that 

 it has in early stages in the Chelonian. 



The details of amnion formation are readily observable in the 

 Fowl and have been fully described by Hirota (1894). The process 

 takes place as follows : The first step consists in the appearance of a 

 crescentic upgrowth of blastoderm just in front of the head of the 

 embryo at about the stage of 14 segments. At this period the meso- 

 derm has spread forwards on each side but has not yet extended into 

 the space immediately in front of the embryonic head (proamnion). 

 Where the mesoderm is present it has split to form the coelome and 

 owing to this being filled with secreted fluid the somatopleure 

 bulges up somewhat so as to be conspicuously marked off from the 

 flat proarnniotic area. The amniotic fold makes its appearance just 

 about the anterior boundary of the proamnion. As it increases in 

 height it overlaps the head of the embryo and grows backwards 

 over it as the amniotic hood (Fig. 233). Into the fold the mesoderm 

 and coelomic cavities have already penetrated. Where the mesoderm 

 from the two sides meet in the mesial plane of the hood the t\v< 

 portions of coelome do not open freely into one another but remain 

 separated by a septum of mesoderm the mesodermal sero-amniotic 

 isthmus. At an early period of the backgrowth of the umniotic 

 hood the ectoderm in the middle of its free posterior ed^v is 

 seen to project head wards as a small wedge, the base of which is 

 formed by the growing edge. As this wedge is carried backwards 

 by the continued progress of the amniotic edge it leaves behind it 

 a kind of trail in tin- form of a continuous line, or rather partition, 

 of ectoderm conned im_r the ectoderm on the outer surface of the 

 amniotic fold with that on its inner surface. This is clearly the 

 ectodermal sero-anmiot ie, isthmus of the K'epi ile ]iersistin^ in a much 

 attenuated form: the ;it feiiuat ion hein^ due to the fact that the 

 ooelomic <-nded much nearer to the mesial plane 



than in the corresponding sia^c of a muion formation in the L'epiile. 



Ip till about the time when Ihe amniofic hood has completed 



its backgrowth it- oavity the amniotie codome remains divided 



