We have already seen in the very young bud, when 

 it consists nerely of two sinple layers, and before there is 

 the slightest indication of the appearance of any organ, that 

 mesodermal cells are attached in many places to the inner 

 surface of the ectoderm, and outer surface of the endoderm, 

 but that they are nowhere more numerous at any one spot than 

 another, Pigs. 7 and 8. Tneir poY,rer of amoeboid movement 

 over any surface v/ith v/hic;T they come in contact, would ac- 

 count for their presence on the walls of the vesicles. 



It would seen more probable that these cells are 

 all alike and undifferentiated, and that the nature of the 

 organs to which they give rise is determined, not by any pre- 

 arranged condition of their idioplasm, but by the particular 

 point to which they happen to become attached. I regard it 

 as a significant fact that mesodermal cells are found, not 

 only at the places v/here organs will arise, but also at many 

 other points. Those of the cells which chance to fall, as 

 it were, on fertile soil, will undergo further development, 

 and under the formative influence exerted upon them by that 

 portion of the wall to which they adhere, will be utilised in 

 building up a definite structure. 



All parts of the walls cannot possess a specific 

 determining po'.ver, and such cells as lodge on barren ground, 

 are not further modified, and do not furnish material for the 

 formation of organs. 



According to this viev/, one mesodermal cell is the 

 equivalent of any otner, and it is only a few that find fa- 



