vin DEVELOPMENT OF AMNION 469 



somatic mesoderm spreads with it. Except along the middle line 

 the amniotic roof thus becomes double — the inner roof being formed 

 of ectoderm internally and somatic mesoderm externally, the outer 

 roof of somatic mesoderm internally and ectoderm externally. Of 

 these two roofs the inner is the amnion (Fig. 214, B, am), the outer 

 is the false amnion or serous membrane (f.a). The ■ portion which 

 retains its original condition of being formed of unsplit ectoderm 

 (sa) may be called the amniotic isthmus or the sero-amniotic con- 

 nexion (Mitsukuri). During later stages of development this becomes 

 reduced to a thin vertical partition in which form it persists through- 

 out, except in the region of the head where it disappears entirely so 

 that there is here a continuous coelomic space stretching from side 

 to side between amnion and serous membrane. 



The posterior tubular prolongation of the amniotic cavity becomes 

 obliterated through part of its extent and in this way the amniotic 

 cavity becomes completely closed. 



The first-formed part of the amnion, lying in front of the head 

 of the embryo, remains for a time proamniotic in character, i.e. 

 composed" of ectoderm and endoderm, but eventually the meso- 

 derm and coelomic space spread in between the two primary 

 layers and the portion of the amnion in question comes to resemble 

 the rest. 



As the body of the embryo becomes constricted off from the yolk- 

 sac the basal edge of the amnion continuous with that of the em- 

 bryonic somatopleure becomes tucked inwards so that the amnion, 

 which formed in earlier stages a mere roof, comes to form a 

 complete envelope. The amniotic cavity is filled with secreted fluid 

 in which the body of the embryo floats. 



Birds.— The process of amnion formation in the Birds shows con- 

 spicuous differences from that which has been described for the more 

 primitive Eeptiles. Two of the chief of these differences seem to be 

 associated with the fact that the amnion develops relatively later in 

 the Bird, at a period when the head and anterior body region of the 

 embryo project prominently above the general level of the blasto- 

 derm and when the mesoderm has already split into splanchnic and 

 somatic layers. Correlated with this fact we find (1) that in the 

 Bird the amniotic rudiment has to grow upwards so as to surround 

 the projecting head and trunk, and (2) that the upgrowth is com- 

 posed of somatopleure only. 



The amnion may be said to originate as a kind of wall, formed of 

 an upwardly projecting fold of somatopleure, which comes to surround 

 . the actual body of the developing embryo. This wall is not abso- 

 lutely vertical : it is tilted, or inclined inwards, towards the middle of 

 the embryonic body. With increasing growth it projects more and 

 more over the body of the embryo, its free edge bounding a gradually 

 diminishing opening, through which the body of the embryo is visible 

 when looked down upon from above. Eventually this opening is 

 reduced to vanishing point and the body of the embryo is completely 



