EARLY STAGES OF DEVELOPMENT OP THE RABBIT. 135 



tion of some which subsequently come under the influence of 

 tension, to which I shall refer again. 



CHAPTER III. 



The Apparent Extension of the Hypoblast. 

 The Fifth Day (97th to 130th hours). 



In the description of the shape and growth of the blasto- 

 dermic vesicle, and the discussion which follows, I call the 

 part of the blastodermic vesicle to which the inner mass is 

 attached the upper pole, the part immediately facing this and 

 furthest removed from it the lower pole. The zone midway 

 between the poles I call the equator. 



The part whereat the central portion of the inner mass is 

 attached to the outer layer, that is to say that part where 

 there are three layers of cells, I call the embryonic disc, but I 

 do not thereby mean to say that only that area and none other 

 takes part in the formation of the adult. 



The events of the fourth day are concerned almost entirely 

 with the enlargement of the embryo. 



During the fifth day this enlargement continues, but other 

 important developments now take place. 



It is during this day that we can first detect a separation into 

 the two well-known embryonic layers, epiblast and hypoblast, 

 and as to the mode of origin of these layers I fear I cannot 

 quite agree with the description and explanations of any former 

 writers on the subject. As 1 shall point out in a future paper, 

 I can see no necessity for the occurrence of any folding in or 

 growing in of cells from without, usually known by the term 

 " gastrulation,^' and most certainly I can find no evidence of 

 it whatever. As far as I know, only one investigator has stated 

 that he has found actual evidence for this in the case of the 

 rabbit. This is Keibel, in his work " Zur Entwickelungs- 

 geschichte der Chorda bei Saiigern," ' Arch, fiir Anat. uud 

 Physiol./ 1889. In this Keibel only found his " blastopore " 



