108 WALTER HEAPE. 



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. Yan 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 KoUiker, 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, hypo- 

 blast, and mesoblast, in the stage of development 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 layer 

 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 upon the dog and mole, in which he states 

 that the epiblast of the embryonic 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 overlies 

 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 blastodermic cavity until the formation of 

 the medullary groove ; an examination of which leads me, in 

 the main, to agree with Lieberkiihn's account of the develop- 

 ment of the embryonic layers of that animal. 



I have not been able to follow completely the course of 

 segmentation, nor have I been able to trace a differentiation 

 of the segments into two layers, an outer and an inner, 

 though there appears to be no doubt, on account of the 

 arrangement of the spheres in a somewhat later stage, that 

 Van Beneden's description of a fully segmented ovum is 

 substantially correct. 



