COMPARISON OF THE GERMINAL LAYERS. 309 



embryology that the blastopore of Vertebrates is primitively situated at the 

 hind end of the body, but that, owing to the development of a large food-yolk, 

 it also extends, in most cases, over a larger or smaller part of the ventral 

 side. 



The origin of the Allantois and Amnion. 



The development and structure of the allantois and amnion have already 

 been dealt with at sufficient length in the chapters on Aves and Mammalia ; 

 but a few words as to the origin of these parts will not be out of place here. 



The Allantois. The relations of the allantois to the adjoining organs, 

 and the conversion of its stalk into the bladder, afford ample evidence that it 

 has taken its origin from a urinary bladder such as is found in Amphibia. 

 We have in tracing the origin of the allantois to deal with a case of what 

 Dohrn would call 'change of function.' The allantois is in fact a urinary 

 bladder which, precociously developed and enormously extended in the em- 

 bryo, has acquired respiratory (Sauropsida) and nutritive (Mammalia) func- 

 tions. No form is known to have been preserved with the allantois in a 

 transitional state between an ordinary bladder and a large vascular sack. 



The advantage of secondary respiratory organs during foetal life, in addi- 

 tion to the yolk-sack, is evinced by the fact that such organs are very widely 

 developed in the Ichthyopsida. Thus in Elasmobranchii we have the 

 external gills (cf. p. 62). Amongst Amphibia we have the tail modified to be 

 a respiratory organ in Pipa Americana ; and in Notodelphis, Alytes and 

 Cascilia compressicanda the external gills are modified and enlarged for re- 

 spiratory purposes within the egg (cf. pp. 140 and 143). 



The Amnion. The origin of the amnion is more difficult to explain 

 than that of the allantois ; and it does not seem possible to derive it from 

 any pre-existing organ. 



It appears to me, however, very probable that it was evolved pari passu 

 with the allantois, as a simple fold of the somatopleure round the embryo, 

 into which the allantois extended itself as it increased in size and became a 

 respiratory organ. It would be obviously advantageous for such a fold, hav- 

 ing once started, to become larger and larger in order to give more and more 

 room for the allantois to spread into. 



The continued increase of this fold would lead to its edges meeting on 

 the dorsal side of the embryo, and it is easy to conceive that they might then 

 coalesce. 



To afford room for the allantois close to the surface of the egg, where 

 respiration could most advantageously be carried on, it would be convenient 

 that the two lamina? of the amnion the true and false amnion should then 

 separate and leave a free space above the embryo, and thus it may have 

 come about that a separation finally takes place between the true and false 

 amnion. 



This explanation of the origin of the amnion, though of course hypothe- 

 tical, has the advantage of suiting itself in most points to the actual ontogeny 



