202 RICHARD ASSHETON. 



Static pressure, it seems probable that the immediate effect of 

 this rapid proliferation of cells at one spot would be to produce 

 at first a mass of cells or a knob, as in reptiles, and then a 

 wrinkling of the thin wall of the vesicle, and eventually a pro- 

 jection or outgrowth. This is, indeed, what takes place a little 

 later, when, by reason of the attachment to the uterus of the 

 walls of the blastodermic vesicle immediately surrounding 

 the embryo, the circumstances are such as to fulfil these condi- 

 tions of equilibrium. The result at this time is that a definite 

 projection and rolling over is effected — the tail fold. 



But as yet the blastodermic vesicle is unattached and lies 

 freely within the cavity of the uterus, but is continually ex- 

 panding in size as described before, owing to the combined 

 effect of increasing hydrostatic pressure and multiplication of 

 the cells of its walls. 



I believe that the conversion of the almost circular spot in 

 fig. 3, or pyriform area in fig. 3, to the linear expression of figs. 

 6 and 7, and back again to the pyriform area of fig. 10, is inti- 

 mately connected with two facts : 



(i) Growth of the embryo taking place from different centres, 

 (ii) Expansion of the whole area, due in this case (rabbit) to 

 the hydrostatic pressure within the blastodermic vesicle. 



In a former paper, " A E-e-investigation into the Early Stages 

 of the Development of the Rabbit," I ascribed the apparent 

 growth round the hypoblast as being due to its being carried 

 round by the outer wall, due to the combined effect of the 

 greater area of special activity around the embryonic disc, 

 and increasing hydrostatic pressure within. May not a 

 similar explanation be given to account for the apparent 

 growth outwards of the primitive streak mesoblast from the 

 primitive streak ? In this case the area of special activity is 

 much more concentrated and vigorous than in the former, and 

 is therefore itself evident as an area of rapid growth. All the 

 cells towards its periphery will be as liable to be removed by 

 the expansion of the walls as the outer layer cells, as with 

 them the inner mass cells towards the periphery are, according 

 to my hypothesis^ apparently removed in the former case. 



