450 WALTER HE APE. 



A consideration of these facts^ together with an examination 

 of the conditions attending the later stages of development in 

 some of these animals, leads me to believe that the difference 

 in the development of normal animals and those in which the 

 so-called inversion of the layers takes place is one of secondary 

 importance, and, in fact, that no such fundamental differences 

 exist as was supposed by the older observers ; the temporary 

 inversion of the layers which occurs in the Mole connecting 

 the two types very closely. 



I do not propose here to enter into a more detailed discus- 

 sion of the points noticed above, or to attempt to compare the 

 later stages of development ; the only points which have imme- 

 diate bearing upon my present work are — 



(1) The explanation of the existence of the secondary cavity 

 and the cells situated within it in the Mole ; and (^) the fact 

 that in the inverted types the epiblast of. the embryo is formed 

 entirely of inner mass cells. 



The former may be consideied as inversion phenomena of a 

 temporary nature; while with regard to the second point the 

 conditions of development appear to me to be sufficient expla- 

 nation of the difference. 



Bibliography. 



(1) F. M. Balfour. — " A Comparison of tlie Early Stages in the Develop- 



ment ot Vertebrates," ' Quarterly Journal of Microscopical Science,' 

 1875. 



(2) r. M. Balfour and F, Deighton. — " A Renewed Study on the Germinal 



Layers of the Chick," ' Quarterly Journal of Microscopical Science,' 

 1882. 



(3) F. M. Balfour. — ' Comparative Embryology,' vol. ii, 1881. 



(4) Edward van Beneden. — "La Maturation de I'oeuf, la fecondation, et les 



premieres phases du dcveloppement Embryonnaire des Maaimiferes 

 d'apres des recherches faites Chez le Lapiu," * Bulletins de I'Academie 

 Royale de Belgique,' 1875. 



(5) Edward van Beneden. — " Recherches sur I'embryologie des Mammiferea. 



La formation des feuillets Chez le Lapin," ' Archives de Biologic,* tome 

 i, fascl. 1, 1880. 



(6) Th. L. W. Bisciioff. — ' Entwickluugsgeschichte des Kanincheu Eies, ' 



Braunschweig, 181'2. 



