COMPARISON OF THE GERMINAL LAYERS, 257 



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 laminae of the amnion — the true and false am- 

 nion — 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 hypo- 

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

 ontogeny of the organ. The main difficulty is the early development of 

 the head-fold of the amnion, since, from the position of the allantois, it 

 might have been anticipated that the tail-fold would be the first formed 

 and most important fold of the amnion. 



Bibliography. 



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(240) F. M. Balfour. "A monograph on the development of Elasmobranch 

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(241) F. M. Balfour. "On the early development of the Lacertilia together with 

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(242) A. Gotte. Die Entwicklungsgeschichte d. Unke. Leipzig, 1875. 



(243) W. His. "Ueb. d. Bildung d. Haifischembryonen." Zeit. f. Anat. u. Ent- 

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(246) C. Kupffer. "Die Entstehung d. Allantois u. d. Gastrula d. Wirbelthiere." 

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(247) R. Remak. Untersuchungen iib. d. Entwicklung d. Wirbelthiere, 1850 — 1858. 



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B. E. II. 17 



