vni DEVELOPMENT OF AMNION 471 



into two separate halves by a septum, which in front is purely 

 mesodermal but throughout the rest of its extent is traversed by 

 the ectodermal sero-amniotic isthmus. The anterior, purely meso- 

 dermal, part of the septum disappears early in the fourth day so 

 as to make the amniotic coelome continuous from side to side, 

 but the rest of the septum persists throughout the whole period 

 of development although its central ectodermal portion becomes 

 gradually reduced and by the tenth day has completely disappeared. 

 Towards the end of the second or early in the third day the tail 

 of the embryo begins to project, bending ventrally and dipping 

 downwards as it does so. As it does this the tail comes to be hidden 

 under a projecting amniotic fold precisely as happened at the 

 head end except that here the coelomic cavity is already completely 

 continuous across the mesial plane there being no trace of a septum 

 or sero-amniotic isthmus. The free edge of this " tail fold " of the 

 amnion is, as was that of the " head fold," concave only here the 

 concavity is directed headwards. Early in the fourth day the 

 concave edges of the head and tail folds become continued into one 

 another at about the level of the hind limb rudiment, so that the 

 body of the embryo is now surrounded by a continuous amniotic 

 fold — most highly developed anteriorly where it forms the amniotic 

 hood, less so in the caudal portion and least of all laterally. The 

 more or less elliptical opening bounded by this fold, through which 

 the dorsal surface of the embryo is exposed, gradually shrinks as 

 the fold grows and eventually, during the first half of the fourth 

 day as a rule, it becomes obliterated and the amniotic cavity closed. 



' The true amnion at first closely ensheaths the head and trunk 

 of the embryo but from about the fifth day onwards watery 

 amniotic fluid is secreted into its interior so as to form an extensive 

 water jacket in which the embryo is suspended (Fig. 215). For a 

 considerable period the embryo is gently rocked to and fro in the 

 fluid by the slow rhythmic contractions of muscle fibres which 

 develop in the somatic mesoderm covering the amnion on its 

 outer surface. 



.The development of the amnion in the Sauropsida in general is 

 adequately illustrated by the two types which have been described. 

 There occur variations in detail. Thus the inequality in the activity 

 of growth between the anterior and posterior portions of the 

 amniotic fold so marked as a rule may be practically absent 

 (Chameleons), or it may reach an extreme limit, the posterior 

 portion of the fold being obsolete and the anterior portion continuing 

 its backgrowth past the tail end of the embryonic body to form an 

 amniotic tunnel, as in the Chelonians above described (Sphenodon, 

 Gannet — Sula, Puffin — Fratercula). 1 



Allantois. — The allantois may also be conveniently studied in 

 the Bird. In the Fowl it makes its first appearance as a little clear 



1 Schauinsland, 1906". 



