FORMATION OF PRO-AMNION. 



35 



extended to this part. The head end of the embryo grows forward over this bilaminar portion, 

 and since the embryo, as it becomes differentiated, tends to sink below the general surface of 

 the blastoderm, the head which now overlies the bilaminar part produces a depression of this 

 part towards the interior of the vesicle, so that the head of the embryo becomes enclosed by 

 the bilaminar wall of the depression (fig. 3(5). The enclosing membrane, which is well marked 

 in the rabbit, has been termed by v. Beneden thepro-amwo : by an extension of mesoblast and 

 of the mesoblastic cleavage between its layers, it afterwards becomes split into somatopleure 

 and splanchnopleure, and the former becomes continuous with the true amnion (see p. 42). 

 The stage of pro-amnion, if it exists at all. must disappear very early in the human embryo. 



Soon after the appearance of the anterior limiting sulcus, two lateral limiting 

 sulci are seen running external and parallel to the medullary folds ; these lateral 

 sulci, as they dip down, mark off the body of the embryo from the rest of the 



Fig. 36. DIAGRAMMATIC LONGITUDINAL SECTIONS THROUGH THE EMBRYO OP THE RABBIT. THE 



SECTIONS SHOW THE MANNER IN WHICH THE PRO-AMNION IS FORMED BY A DIPPING DOWN OP 

 THE HEAD AND ANTERIOR PART OP THE BODY INTO A DEPRESSION OP THE BLASTODERM, WHICH 

 AT THIS PART IS FORMED OF EPIBLAST AND HYPOBLAST ONLY. THE DIAGRAMS ALSO ILLUSTRATE 

 THE MODE OF FORMATION OF THE ALLANTOTS AND OP THE TAILFOLD OF THE AMNION IN THIS 



ANIMAL, (v. Beneden and Julin.) 



ep, epiblast ; A?/, hypoblast ; me, mesoblast; cce, parts of the coelom ; cw', pericardia! coelom, the 

 heart not being represented ; pr.a., pro-amnion ; pi, seat of formation of the placenta ; all. allantois ; 

 am, amnion. 



blastoderm, but they do not for some time progress far in development, the middle part 

 of the future alimentary tract long remaining in free continuity with the cavity of the 

 blastodermic vesicle (fig. 45, and fig. 49, (T), but becoming gradually more pinched off 

 from it. That part of the cavity of the original blastodermic vesicle which does not. 

 form a part of the alimentary canal, but remains connected with it by a wide neck 

 of communication, is known as the yolk-sac. At a later stage, when the 'body walls 

 are formed, and the yolk-sac, relatively greatly diminished in size, lies altogether 

 outside the body of the foetus, it is merely connected by a long narrow duct, which runs 



D 2 



