522 W. F. LOOMIS 



motion results secondarily from the protoplasmic streaming that was 

 originally designed to aerate even the innermost cytoplasm of a cell. With 

 a cell as enormous as an amoeba, this is, of course, more necessary than 

 usual. 



Gradients of pCOo are highly dependent on simple geometrical forces 

 such as those of total mass, flattened versus spherical shape and similar 

 changes in the surface /volume ratio of an aggregate of cells. A delightful 

 description of the fundamental character of such geometrical forces is 

 presented in John Bonner's Morphogenesis [14]. His conclusion is in- 

 escapable: i.e. nature uses these simplest of all considerations to build up 

 progressive complexities during the development of an embryo. The 

 enormous difference between the fate of a blastomere that is separated 

 from other cells and one that is left attached to another blastomere, be it 

 alive or dead, is a case in point. Even a dead blastomere somehow affects 

 its living twin by its mere physical presence, an effect that can at least 

 speculatively be assigned to a distortion of the gradient fields of pCOo 

 produced in the living half. 



If embryonic differentiation is to proceed along a pCOg gradient of 

 the type Rachevsky pictured, it must be permanent and not upset by 

 cytoplasmic streaming as in the body of an amoeba. One means of stabil- 

 izing such a gradient is to have the egg cleave progressively into smaller 

 and smaller cells so that the protoplasm at the centre of the mass is locked 

 in place along the over-all gradient that extends throughout the entire 

 mass of respiring cells (Fig. 5). Looked at from this angle, it is not sur- 

 prising that cell cleavage is the first order of business in the developing 

 embryo, for it prevents cytoplasmic streaming by the erection of cell 

 membranes through which COg molecules may travel but behind which 

 the protoplasmic contents of each cell is locked in place. According to this 

 view, a physico-chemical gradient of pCOg would first form as a result 

 of the respiration of the cells themselves. Only gradually would this 

 chemical gradient be transformed into structurally different cells that 

 differentiated in each location according to the micro-environmental level 

 of pCO., that existed at that particular site. 



John Bonner has emphasized that the embryo uses cell movement as 

 well as cell growth and differentiation to achieve its ends (Fig. 6). Here 

 we encounter such problems as (i) the acrasin phenomenon in which 

 slime-mould amoebae become mutually attractive to each other; (2) why 

 the dorsal lip of the blastopore grows downward and into the hollow sphere 

 of the blastula when it might well grow outward into new space as during 

 budding; (3) why certain epithelial cells sink down below the surface as 

 during the formation of the neural groove. 



If cell migration is vital to embryogenesis, perhaps it is because it takes 

 cells that have been programmed one way and then exposes them to another 



