396 JAS. P. HILL. 



tersackclioriou seu pseudo-choriou " (Selenka), are open to 

 objection. Hubrecht's term " omphaloidean diplotrophoblast " 

 (9, p. 385) is also inadequate, since it is only properly appli- 

 cable to the somatopleural constituents of the as yet unsplit 

 yolk-sac wall. To avoid confusion it seems desirable to 

 employ a distinctive term descriptive of this unsplit yolk-sac 

 wall in its entire thickness and extent ; and for this end I 

 propose to adopt, at the suggestion of my friend Professor 

 J. T. Wilson, the term " omphalopleure." 



The terra omphalopleure, then, signifies the whole 

 of the wall of the blastodermic vesicle or primitive 

 yolk-sac, beyond the region of extension of the 

 splanchnoccele. The employment of the term omphalo- 

 pleure will thus prevent the unnecessary use of the expression 

 'Milastodermic vesicle walP' in stages when the embryo is 

 completely folded off, and one no longer wishes to speak of a 

 blastodermic vesicle as such. According to the extension of 

 the unsplit mesoderm the omphalopleure may be trilaminar or 

 bilaminar in greater or lesser extent. Also a unilaminar con- 

 dition may be temporarily found in a position corresponding 

 to the lower pole of the blastodermic vesicle, prior to com- 

 plete ventral extension of the yolk-sac entoderm. 



In Marsupials the trilaminar portion of the omphalopleure 

 is co-extensive with the vascular area, the sinus terminalis 

 marking not only the margin of the vascular area, but also the 

 peripheral limit of the unsplit mesoderm. We may thus refer 

 to the trilaminar omphalopleure in Marsupials as the " vascular 

 omphalopleure''^ (text fig., vase. omph.). Beyond the sinus 

 terminalis mesoderm is absent, and there the omphalopleure 

 consists solely of ectoderm and entoderm (text fig., bil. 

 omph.). We shall hereafter refer to this as the *' bilaminar 

 omphalopleure," which seems more expressive than Semon's 

 term '' Prokalymma " (8). We can thus distinguish in the 

 outer wall limiting the whole spheroidal embryonic formation 

 three areas of widely different structure : towards each pole a 

 discoidal area, — the one limiting the extra-embryonic coelom, 

 and consisting of true chorion ; the other limiting the yolk-sac 



