SEGMENTATION OF THE OVUM OP THE SHEEP. 255 



development of Mammalia is presented by the epibolic gas- 

 trula and tiie formation of the blastodermic vesicle. . . . No 

 satisfactory phylogenetic explanation of the mammalian 

 gastrula has^ in my opinion, as yet been offered. ^' 



Although my inferences as to the homologies of the amniotic 

 folds cannot be said to have been contemplated by the writer of 

 these words, I hope it may be thought that my work upon the 

 segmentation of the sheep might have removed from his mind 

 the difficulty of the supposed epibolic gastrula of the mammal 

 by showing that such a phenomenon does not occur, and that the 

 appearance which has given rise to the misconception is capable 

 of quite a different interpretation in complete accord with his 

 view that the development of the maminalian ovum bears 

 traces of a descent from an ancestor with a large-yolked egg. 



References. 



1. AssHETON, R. — " A Re-investigation into tiie Early Stages of the Deve- 



lopment of tiie Rabbit," ' Quart. Journ. Micr. Sci.,' vol. xxxvii, 1894. 



2. AssHETON, R. — " On the Phenomenon of the Fusion of the Epiblastic 



Layers in the Rabbit and in the Frog," ' Quart. Journ. Micr. Sci.,' 

 vol. xxxvii, 1894. 



3. AssHETON, R. — " The Primitive Streak of the Rabbit ; the Causes which 



may determine its Shape, and the Part of the Embryo formed by its 

 Activity," ' Quart. Journ. Micr. Sci.,' vol. xxxvii, 1894. 



4. AssHETON, R. — " An Experimental Examination into the Growth of the 



Blastoderm of the Chick," ' Proc. Roy. Soc.,' vol. Ix, 1896. 



5. AssHETON, R. — " The First Ten Days of the Development of the Pig," 



(in the press) ' Quart. Journ. Micr. Sci.,' the present volume, 1898. 



6. Baleotje, F. M. — ' Comparative Embryology,' 2nd edit., London, 1885. 



7. VON Baek, K. E. — ' Ueber Entwickelungsgeschichte der Thiere,' Kon- 



igsberg, 1828. 



8. VAN Beneden, E. — " De la maturation de I'ceuf, de la fecondation et des 



premiers pheuomenes embryonnaires chez les Mammiferes d'apres les 

 observations faites chez le lapin," 'Bulletins de 1' Academic Royale 

 des Sciences de Belgique,' 1875. 



9. VAN Beneden, E. — "Recherches sur I'embryologie des Mammiferes, la 



formation des feuillets chez le lapin," ' Archives de Biologic,' tome i 

 1880. 



