852 



SCIENCE 



[N. S. Vol. XXXVI. No. 938 



visible structure is concerned, are the em- 

 bryonic equivalent of tissues in the adult. 

 These germ layers almost immediately 

 undergo local modifications wherein we see 

 the embryonic equivalent of organs. And 

 before development progresses very far, 

 structural peculiarities appear which corre- 

 spond to specific characteristics of the 

 whole individual. As the observer attempts 

 to follow these events of ontogeny he soon 

 finds himself confused and lost in so great 

 a complication of developmental opera- 

 tions that he can no longer surely distin- 

 guish processes which are functions of the 

 organization of cells as cells, and processes 

 which are functions of the organization of 

 embryonic tissues, and others which may 

 be functions of units of organization of yet 

 higher grade. 



In our approach toward the problem of 

 organization, a step of the utmost impor- 

 tance will have been gained when we have 

 so far analyzed developmental operations 

 that each component process may be posi- 

 tively identified as the function of a struc- 

 tural unit corresponding to an organiza- 

 tion of a certain grade. We must at the 

 outset clearly distinguish between processes 

 which depend upon the operation of proto- 

 plasmic mechanism of one grade or another, 

 and those events or conditions which are 

 entirely independent of active physiolog- 

 ical factors, as, for example, Avlien the orien- 

 tation of an egg cell in space and the dis- 

 tribution of some substance in the egg are 

 directly due to gravity acting upon a heavy 

 yolk. We must then determine, for each 

 truly physiological developmental event, its 

 value or position in the scale of organiza- 

 tions. We shall then have come to recog- 

 nize in a certain developmental event, for 

 example, a process which requires the 

 operation of no organization higher than 

 that which is fundamentally inherent in all 

 cells. Any cells whatever, being in that 



particular physiological state — that is, as 

 I'egards general metabolic conditions and 

 the like — and placed in that particular 

 physical environment, would exactly re- 

 produce the developmental event which is 

 before us. (I distinguish between the 

 physical environment which includes all 

 those conditions, such as temperature, pres- 

 sure, chemical constitution of the medium, 

 which are either independent of the struc- 

 ture of protoplasm, or only indirectly or 

 remotely determined by it ; and the physio- 

 logical environment, essentially physical in 

 nature, which includes the action of im- 

 mediately present specific protoplasmic 

 mechanisms.) Thus, it is conceivable that 

 the earlier cleavage events, in at least some 

 animals, are of this elementary character, 

 inasmuch as they may show no definite or 

 necessary relations to the organizations 

 which appear later. In many eases the 

 cleavage plan may be profoundly modified 

 without important effect upon the subse- 

 quent development. In another instance 

 we may distinguish a process for which the 

 mechanism common to all cells is not ade- 

 quate. It is, we will suppose, a process re- 

 quiring, superimposed upon the essential 

 cell mechanism, something else which re- 

 sults in the association together of numer- 

 ous similar cells to constitute a layer or 

 tissue of specific structural and physiolog- 

 ical character; for example, the ectoderm. 

 Beyond that in this case we need not go, 

 for it becomes clear to us that any group 

 of cells Avhatever, belonging to a tissue of 

 this type and placed in a physical situation 

 like that which exists in our supposed case, 

 would exhibit that particular formative be- 

 havior which we have there observed. It 

 is the essential point in this case that the 

 process is one which has no necessary rela- 

 tion to the specific features of an organ or 

 any higher complex of the individual. It 

 may, indeed, be a process which partici- 



