130 RICHARD ASSHETON. 



This, though rather a rough method of investigation, shows 

 sufficiently well that there is a proportionally greater rapidity 

 of increase of the cells on the outside over that of the inner 

 cells. 



Growth of the embryo must surely be dependent upon 

 nourishment from without, when the bulk of the mass increases 

 as it does from the 70th hour. It is the cells upon the outside 

 of the morula which are in the most favourable position for the 

 acquisition of such nourishment from the fluids of the uterus 

 or Fallopian tube. No doubt during the early stages of seg- 

 mentation the energy of division is derived from the nutriment 

 — yolk^ &c. — contained within the ovum itself. As this be- 

 comes used up the embryo will become more dependent upon 

 external sources. This will mean that the externally placed 

 segments will become more favourably placed for growth than 

 the internally placed segments. May not this gradual ex- 

 haustion of intrinsic nutriment be a determining factor in the 

 cessation of the increase of the embryo as a morula and causa- 

 tion of the first commencement of a cavity ? 



This may give rise to the first cleft, but in itself it can 

 hardly account for the large increase in size of the blasto- 

 dermic vesicle. The cells are so very delicate it is hardly con- 

 ceivable that they could cause the great expansion of the very 

 tough albuminous wall. It seems far more likely that the 

 force which causes the expansion is due to an osmotic current 

 being more rapid inwards than outwards^ either simple or 

 more probably assisted by the vital activity of certain cells of 

 the embryo, as is supposed in the case of diffusion in intestine, 

 and suggested by Heape in connection with the mole. Or the 

 diff'usion process may be a simple physical process, but the 

 nature of the liquid after entering the cavity may be so changed 

 by the activity of the cells as to render its diff'usibility less 

 when once inside than before its entrance from the uterus. 



However this may be, the most noticeable fact of the 

 development of the rabbit embryo during the fourth day is the 

 commencement and enlargement of the cavity of the blasto- 

 dermic vesicle. 



