220 FORMATION OF THE LAYERS. 



stituent cells increase in number and become one by one colum- 

 nar; and that in the process they press against the layer of 

 rounded elements below them, so that the two layers cease to be 

 distinguishable, and the whole embryonic area acquires in section 

 the characters represented in fig. 136'. Towards the end of the 



FIG. 136. SECTION THROUGH THE NEARLY CIRCULAR EMBRYONIC AREA OF A 

 RABBIT'S OVUM OF six DAYS, NINE HOURS AND -8 MM. IN DIAMETER. 



The section shews the peculiar character of the upper layer with a certain number 

 of superficial flattened cells; and represents about half the breadth of the area. 



sixth day the embryonic area becomes oval, but the changes 

 which next take place are not understood. In the front part of 

 the area only two layers of cells are found, (i) an hypoblast, and 

 (2) an epiblast of columnar cells probably derived from the 

 flattened epiblast cells of the earlier stages. In the posterior 

 part of the blastoderm a middle layer is present (Van Beneden) 

 in addition to the two other layers; and this layer probably 

 originates from the middle layer which extended throughout the 

 area at the beginning of the fifth day, and then became fused 

 with the epiblast. The middle layer does not give rise to the 

 whole of the eventual mesoblast, but only to part of it. From its 

 origin it may be called the hypoblastic mesoblast, and it is 

 probably equivalent to the hypoblastic mesoblast already de- 

 scribed in the chick (pp. 154 and 155). The stage just described 

 has only been met with by Van Beneden 2 . 



A diagrammatic view of the whole blastodermic vesicle at 

 about the beginning of the seventh day is given in fig. 137. The 

 embryonic area is represented in white. The line ge in B shews 

 the extension of the hypoblast round the inner side of the vesicle. 

 The blastodermic vesicle is therefore formed of three areas, (i) 



1 The section figured may perhaps hardly appear to justify this view; the exami- 

 nation of a larger number of sections is, however, more favourable to it, but it must 

 be admitted that the interpretation is by no means thoroughly satisfactory. 



2 Kolliker does not believe in the existence of this stage, having never met with it 

 himself. It appears to me, however, more probable that Kolliker has failed to obtain 

 it, than that Van Beneden has been guilty of such an extraordinary blunder as to have 

 described a stage which has no existence. 



