256 ORIGIN OF ALLANTOIS AND AMNION. 



Vertebrates the ventral walls of the body are obviously completed by the 

 closure of the lips of the blastopore, on the ventral side. 



If His and Rauber are right the dorsal walls are also completed by the 

 closure of the blastopore, so that the whole of the dorsal, as well as of the 

 ventral wall of the embryo, must be formed by the concrescence of the lips 

 of the blastopore ; which is clearly a reductio ad absurdum of the whole 

 theory. To my own arguments on the subject I may add those of Kupffer, 

 who has very justly criticised His' statements, and has shewn that growth 

 of the blastoderm in Clupea and Gasterosteus is absolutely inconsistent 

 with the concrescence theory. 



The more the theory of His and Rauber is examined by the light of 

 comparative embryology, the more does it appear quite untenable; and it 

 may be laid down as a safe conclusion from a comparative study of verte- 

 brate 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. 



Tlie 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 

 embryo, has acquired respiratory (Sauropsida) and nutritive (Mammalia) 

 functions. 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 fcetal life, in 

 addition 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. 51). Amongst Amphibia we have the tail modified 

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

 and Cpecilia compressicanda the external gills are modified aud enlarged 

 for respiratory purposes within the egg (cf. pp. 116 and 118). 



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 part 

 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, having 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. 



