vin DEVELOPMENTAL ADAPTATIONS 475 



penetrates the air-space and pulmonary breathing begins. The 

 allantoic circulation then gradually becomes sluggish and stops, and 

 eventually by a process of autotomy the allantois is separated from 

 the body of the embryo and is left behind as the vascular membrane 

 seen lining the fragments of shell from which a young bird has 

 hatched. 



Enclosure of Yolk-sac within the Embryonic Body. — As 

 already indicated the yolk-sac becomes eventually (about a couple of 

 days before hatching in the 'case of the common Fowl) enclosed 

 within the body-wall. The process by which this is brought about 

 appears to be as follows (H. Yirchow). With the growth of the 

 embryo a great increase takes place in the area over which the 

 amnion is fused with the proximal wall of the allantois (cf. Pig. 

 215a, C), the compound and highly muscular membrane so formed 

 extending eventually almost completely round the yolk-sac. At its 

 edge it is continued onwards by the somatopleure, this latter termin- 

 ating round the circular area where the yolk remains exposed. The 

 'yolk-sac is thus contained in a space the wall of which is formed of 

 the following components in sequence starting from the body of the 

 embryo : (1) amnion, (2) amnion fused with proximal wall of allantois, 

 (3) proximal wall of allantois and (4) somatopleure in the region of 

 the distal pole of the yolk-sac. The proximal portion of this wall, 

 being formed of amnion, is necessarily continuous with the body-wall 

 of the embryo at the umbilical opening and further those parts of it 

 formed from amnion and allantois are highly muscular and con- 

 tractile. During the later stages of development this wall slowly 

 contracts and as it does so the yolk-sac is pushed into the umbilical 

 opening which closes after it. 



Evolutionary Origin of the Amnion. — As regards this question, 

 which has excited much controversy, the following appears to the 

 present writer to be the working hypothesis which fits most easily 

 the facts so far as they are known. 



(1) The amnion originated as a fold of blastoderm round the 

 body of the embryo (Fig. 216, A, B). 



As has already been shown the amnion arises in this way in 

 ontogeny in the Eeptilia which are generally recognized as being the 

 most primitive Amniotes. The same holds for the Birds and for 

 some of the Mammals. 



The Mammalia as a group are admittedly descended from ancestors 

 in which the egg was large and meroblastic as it is in the Eeptilia. 

 This is indicated, apart from other convincing evidence, by the fact 

 that they still exhibit in ontogeny a well-developed though yolkless 

 " yolk-sac." It follows then that it is inadmissible to regard facts 

 derived from the study of certain mammals in which the mode of 

 amnion formation during ontogeny is of a different, even though 

 apparently simpler, type as constituting important evidence in regard 

 to the phylogenetic origin of the amnion, as has been done in par- 

 ticular by Hubrecht (1895). 



