146 EICHAED ASSHETON. 



the 98th to 108th hours or thereabouts in the rabbit, I cannot 

 conceive that the migration can be accomplished otherwise 

 than by an actual migration, for which there is no evidence, or 

 else by a process such as I have attempted to describe above. 

 This I believe to exist. If the inner layer was not composed of 

 isolated cells, but was a compact membrane, then it might creep 

 round by means of its own interstitial growth, although I do 

 not think it would in that case be a thin smooth membrane. 



Such is the account of the growth round of the hypoblast in 

 the mole. Heape makes no mention of any isolated cells at 

 the edges ; he says of the hypoblast, '' it extends laterally by 

 virtue of the multiplication of its cells, which at the same time 

 become much flattened." It seems to me to be much more 

 likely that the layer would be flattened if they were drawn out 

 along with the epiblast cells ; but I have no evidence at present 

 whether the same cause can be attributed to the spread of the 

 hypoblast in the mole as I suggest for the rabbit. 



In fig. 43 the dotted lines radiating from the centre of the 

 smallest blastocyst indicate the amount of growth of the 

 several segments up to the one hundred and seventy-fifth hour. 



Until the one hundredth hour I imagine the growth of all 

 parts of the wall of the blastocyst to be equal. 



From that moment there is a greater activity in an area 

 around the embryonic disc, which causes the inner layer to be 

 apparently carried further round the anterior of the blastocyst. 



This zone of activity by the one hundred and fortieth hour 

 has become more marked still, and now shows itself to be more 

 intense posteriorly than anteriorly, which latter character is 

 plainer still at the one hundred and seventy-fifth hour. 



Eventually the activity culminates, upon the addition of 

 resistance afforded by the walls of the uterus, in the production 

 of the ectoplacental area, or part of it. 



To this I have referred in another paper, and also to the 

 hypothesis that this zone of activity, in the absence of a tough 

 albuminous coat, results in certain other rodents in the pro- 

 duction at once of a heaping up of cells — the trager. 



In the embryonic disc region there is very much less activity 



