470 EMBEYOLOGY OF THE LOWEE VEETEBEATES oh. 



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

 membrane. 



The amniotic fold does not develop with equal activity through- 

 out its extent. Its growth is much more 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. Pigs. 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 proamniotic 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 two 

 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 amniotic 

 hood the ectoderm in the middle of its free posterior edge is 

 seen to project headwards 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 the form of a continuous line, or rather partition, 

 of ectoderm connecting the ectoderm on the outer surface of the 

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

 ectodermal sero-amniotic isthmus of the Eeptile persisting in a much 

 attenuated form; the attenuation being clue to the fact that the 

 coelomic spaces have extended much nearer to the mesial plane 

 than in the corresponding stage of amnion-formation in the Eeptile. 



Up till about the time when the amniotic hood has completed 

 its backgrowth its cavity — the amniotic coelome — remains divided 



