On the GrERMiNAL Layers and Early Development of 

 the Mole. By Walter He ape. 



The following is a note on some investigations which I 

 have been carrying on by the kindness and with the help of 

 Mr. Balfour, in the Morphological Laboratory, Cambridge, 

 upon the origin and formation of the germinal layers in 

 mammals, more especially in the mole [Talpa Europea). I 

 hope shortly to be able to give a more complete account. 



In the communication the following subjects are dealt 

 with: 



(1.) The origin of the epiblast. 



(2.) The mode of development of the mesoblast. 



(3.) The structure of the neurenteric canal. 



(4.) The relations of the mesoblast and the hypoblast to 

 the notochord. 



Recent investigations have left the earlier phases of mam- 

 malian development in some confusion, it may therefore be 

 advisable briefly to mention the more important views which 

 are entertained on this subject. 



Professor Edward van Beneden, in a paper entitled " La 

 formation des feuillets chez de Lapin " (^Archives de 

 Biologic,' vol. i. Part 1, 1880), gives an account of the 

 segmentation of the ovum of that animal, and states that 

 during segmentation a differentiation of the segmentation 

 spheres into two layers is established, the one of which 

 grows over and encloses the other, giving rise in this manner 

 to what Van Beneden calls a metagastrula. The outer of 

 these two layers he terms ectoderm and the inner entoderm, 

 names which seem to me, for reasons which will appear in 

 the sequel, to be misleading, and for which I propose to sub- 

 stitute the terms outer and inner layers respectively. 



Subsequently, according to Van Beneden, a cavity, the 

 blastodermic cavity, is developed between the outer and inner 

 layers of cells ; the cells of the former layer become flattened 

 and multiply, and form the wall of the so-called blastodermic 

 vesicle ; at the same time the blastodermic cavity is enlarged, 



