THE AMNION. 



31 



Serosa 



Ectoblast "I 



/-Amnion 

 Mesoblastj 



Exocoelom 



■Open gut-tube 

 Splanchnopleura 



duplicatures of somatopleura from all sides results in the encircling of the embryo 

 within a wall which increases in height until the prominent edges of the folds meet 

 and coalesce over the dorsal aspect of the enclosed embryo. The folds of the 

 amnion first meet over the head-end, from which point the union extends tailward, 

 where, however, fusion may be delayed for some time. The line along which the 

 iunction of the folds takes place is known as the amniotic suture. 



The amnion thus forms 

 a closed sac completely in- FJG. 37. 



vesting the embryo and con- 

 taining a fluid, the liquor 

 amnii ; at first closely sur- 

 rounding the embryo, the 

 amniotic sac'rapidly expands 

 until its dimensions allow the 

 enclosed foetus to turn freely, 

 practically supported by the 

 amniotic fluid, which pos- 

 sesses a specific gravity of 

 1003. It has long been 

 known that in certain forms, 

 conspicuously in the chick, 

 the amnion executes rhythmi- 

 cal contractions, at the rate 

 of ten per minute, whereby 

 the embryo is swayed from 

 end to end of the sac. From 

 the manner of its formation, 

 as folds of the somatopleura 



(Figs. 37 and 38), it is evident that the amnion consists of an inner ectoblastic and 

 an outer mesoblastic layer. 



The Serosa, or False Amnion. — Coincident with the fusion of the inner 

 layers of the somatopleuric folds to form the closed sac of the amnion, the outer 



layers of the same folds unite to 



Fig. 38. 



Serosa 



Amnion 



frophoblast 



Vitelline sac 



Entoblast 



Diagram showing formation of amniotic folds and of gut-tube ; trans- 

 verse section of axis of embryo. 



mniotic sac 



Gut-tube 



Exocoelom 



Splanchno- 

 pleura 



produce a second external en- 

 velope, the serosa, or false am- 

 nion. The serosa soon becomes 

 separated from the amnion by 

 an intervening space to form the 

 primitive chorion ; the latter, 

 therefore, consists of ectoblastex- 

 ternallyand mesoblast internally, 

 the reverse of the disposition of 

 these layers in the amnion. 



The outer surface of the 

 mammalian primitive chorion — 

 the entire envelope formed of 

 the serosa and the trophoblast 

 — is distinguished by prolifera- 

 tion of the epithelial elements, 

 which process results in the 

 production of more or less con- 

 spicuous projections or villi 

 (Fig. 40), this villous condition 

 being particularly well marked 

 in man. 

 The ectoblast of the primitive chorion takes no part in the formation of 

 the body of the embryo, but, on the other hand, assumes an important role in 

 establishing the earliest connection between the embryo and the maternal tissues 

 and, later, participates in the formation of the placenta. The ectoblast of the 



Vitelline sac 

 Trophoblast 



Entoblast 



Diagram showing formation of amniotic folds and vitelline sac ; 

 longitudinal section of embryo. 



