1881.] and Early Development of the Mole. 191 



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 substitute 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 blasto- 

 dermic cavity is enlarged, while the inner layer remains as a rounded 

 mass of cells attached to the wall of the vesicle over a small area known 

 as the embryonic area. Van Beneden considers that the outer layer of 

 cells forms the permanent epiblast, both of the embryonic area and of 

 the blastodermic vesicle, while the inner mass of cells breaks up into 

 two layers, a lower single layer of flattened cells, the hypoblast, and a 

 layer of cells which he calls the mesoblast, lying between the hypo- 

 blast and the epiblast of the embryonic area. 



Professor Kolliker, on the other hand, writing in the " Zoologischer 

 Anzeiger" (Nos. 61 and 62, vol. iii, 1880), "Die Entwicklung der 

 Keimblatter des Kaninchens," does not dispute the presence of Van 

 Beneden's epiblast, hypoblast, and mesoblast, in the stage of develop- 

 ment described above, but states his agreement with an earlier view 

 •of Rauber, that in the region of the embryo the outer of these layers 

 disappears, while the whole of the middle laj^er becomes converted 

 into the epiblast of the embryonic area ; the epiblast of the remainder 

 of the vesicle, however, he considers is formed from part of the 

 original outer layer of cells. The mesoblast owes its origin, in his 

 opinion, wholly to a budding from the epiblast of the primitive 

 streak. 



Professor Lieberkiihn published in Marburg, in 1879, in a paper 

 ■"Ueber die Keimblatter der Saugethiere," the results of his researches 

 npon the dog and mole, in which he states that the epiblast of the em- 

 bryonic area is derived from the greater part of the primitive inner 

 mass of cells (that portion in fact forming Van Beneden's mesoblast), 

 together with the part of the original outer layer of cells which over- 

 lays the inner mass ; the hypoblast he derives from the inner mass of 

 cells, while the mesoblast he believes to be formed from both epiblast 

 ■and hypoblast, in the region of the primitive streak. 



I myself have been fortunate enough to secure a fairly complete 

 series of mole embryos ranging from an early appearance of the blasto- 

 dermic cavity until the formation of the medullary groove ; an exami- 



