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 lamina? of the amnion the true and false arn- 

 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, froih the position of the allantois, it 

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

 and most important fold of the amnion. 



BIBLIOGRAPHY. 



(239) F. M. Balfour. "A comparison of the early stages in the development of 

 Vertebrates." Quart. J. of Micr. Science, Vol. xv. 1875. 



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

 Fishes." London, 1878. 



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

 some observations, etc." Quart. J. of Micr. Science, Vol. xix. 1879. 



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



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

 wick., Vol. ii. 1877. Cf. also His' papers on Teleostei, Nos. 6$ and 66. 



(244) A. Kowalevsky. "Entwick. d. Ampkioxus lanceolatns." Mem. Acad. dcs 

 Sciences St Petersburg, Ser. vn. Tom. xi. 1867. 



(245) A. Kowalevsky. "Weitere Studien iib. d. Eutwick. d. Arnpnioxus lan- 

 ceolatus." Archiv f. mikr. Anat., Vol. xin. 1877. 



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

 Zool. Anzeiger, Vol. u. 1879, pp. 520, 593, 612. 



(247) E. Bemak. Untersuchungen iib. d. Entwicklung d. Wirbelthiere, 1850 18.38. 



(248) A. Bauber. Primitiustreifen u. Neurula d. Wirbelthiere. Leipzig, 1877. 



E. 11. 17 



