ORGAN-FORMING MATERIALS IN FROG'S EMBRYO. 135 



to indicate that they are due only to the absorption of the blas- 

 toccel fluid, since the cells that undergo the changes in question 

 do not appreciably grow larger than the others. On the con- 

 trary the gastrulation seems to be due to the change in shape and 

 migration of certain cells, and the absorption of the fluid from 

 the blastoccel appears to be no more than the ordinary process 

 of water absorption that takes place throughout the whole period 

 of development. 



The evidence that we have at present seems to indicate that 

 the process of gastrulation is due to the activities of the cells 

 themselves, /. t\, the " mechanics " of the process can only be ex- 

 plained by an appeal to the response of the cells to certain stimuli. 

 The actual changes that we observe involve a change in shape 

 of certain cells, and our problem resolves itself into determin- 

 ing what stimulus leads to this change in shape, and what phys- 

 ical process is involved in the change. I have pointed out that 

 it seems to me probable that the stimulus is derived from the 

 mutual pressures of the cells, and the change of shape is due to 

 contraction processes in the cells (that lead to their change in 

 shape) that are akin to the same contraction shown by a proto- 

 zoon, or, in a higher form, by muscle cells. The different be- 

 havior of the cells in different parts of the egg must be ascribed 

 to their difference in materials that are derived from the different 

 parts of the egg. After each cell-division, itself apparently a 

 process of local contraction, there must be a rearrangement of the 

 pressure relations in the different parts of the egg. 



The formative factors of development can be reduced, from 

 this point of view, to the two generally recognized properties of 

 living matter, irritability and contractility. The stimulus that 

 arouses the irritability is the pressure relation of the cells ; the 

 time at which each cell is effected by this stimulus will depend on 

 its material composition, and it responds by its contraction. If 

 this interpretation of the formative changes is correct it refers the 

 process of development to two physiological properties of living 

 matter, and not directly to other known physical properties of 

 organic matter. What the physical bases of irritability and of 

 contractility are remains for the future to decide. They may be 



