SEGMENTATION OF THE OVUM OF THE SHEEP. 231 



wall. As far as it is possible to judge from these figures there 

 is uothiug to absolutely condemn such an interpretation. 



The Bat. 



An attempt to reinstate van Beneden's (8^ 9) metagastrula 

 theory for the development of the mammalian ovum has quite 

 recently been made by Professor Matthias Duval in his work 

 upon bats in the ' Journal de I'Anatomie et de la Physiologic' 

 for the years 1895, 1896. 



Duval found in the early stage of the development of Ves- 

 pertilio murinus a stage exactly comparable to that which 

 van Beneden described for the rabbit. 



Quite in accordance with van Beneden's earliest account of 

 the segmentation of the ovum in the rabbit, Duval believes 

 that in the bat the whole of the inner cell mass becomes hypo- 

 blast, and the outer layer alone becomes epiblast. Like van 

 Beneden he traces the origin of the distinction between the 

 hypoblast and epiblast to the first division in the process of 

 segmentation of the ovum. 



From the first two segments a group of larger and more 

 slowly dividing cells arises, and forms eventually the whole of 

 the inner mass, which is surrounded by the smaller and more 

 rapidly dividing descendants of the other primary segment. 



The outer layer is from the moment of its first formation a 

 single cell in thickness, and it remains so until a cavity 

 appears between the inner mass of cells and the outer layer, 

 and converts the embryo into a typical blastodermic vesicle. 

 The vesicle expands, and the inner mass is almost entirely 

 flattened out, so that at one time a condition is attained when 

 the vesicle at its upper pole consists of an outer layer of a 

 single cell in thickness — the epiblast, and an inner layer also 

 of a single layer in thickness — the hypoblast. 



The outer layer now begins to thicken, and forms the em- 

 bryonic knob. In this knob an irregular cavity subsequently 

 appears, portions of the roof of which are thrown off and 

 absorbed, and the floor remains as the permanent epiblast. 



This description as regards the segmentation of the ovum. 



