524 W. F. LOOMIS 



as little as 5% CO2 reversibly inhibits aggregation, an inhibition that may 

 be due to the failure of one amoeba to find the COo generated by its 

 neighbour when a high percentage of CO2 is present everywhere and in no 

 relation to the location of other amoebae. This possibility is further 

 supported by the fact that slime mould amoebae will not aggregate in the 

 presence of bacteria — possibly because these bacteria also generate CO2 

 and so drown out the amoeba-to-amoeba message. Like fireflies in the 

 daylight, the background is too high for the "message" to get through. 



In summary, embryology seems surprisingly hospitable to various 

 applications of the pCOo theory. Not only does it shed light, and suggest 

 experiments, on the meaning of egg cleavage, but it continues to do so 

 during gastrulation and the formation of the three germ layers. Formative 

 cell movements connected with the formation of the heart have been 

 shown by Ebert [16] to begin only after a critical cell mass has been 

 achieved. Looked at from the PCO2 point of view, such facts become 

 understandable, for the embryo seems to contain its future structure 

 within itself much as a fireworks rocket exploding in the night sky first 

 releases a burst of blue, then red, followed in turn by yellow and green, 

 the fuse of each setting ofl^ each subsequent explosion. Rather than looking 

 for some structure-giving external organizer such as was pictured in 

 Spemann's day, we can visualize a sequence of micro-environments self- 

 created by the developing egg as it cleaves into a thousand cells and then 

 invaginates to form the three- layered structure of the gastrula. Spemann's 

 induction of a second embryo is not so miraculous as it first appears, for 

 once started, an inevitable chain of events would proceed as they do when 

 activated by the entrance of a sperm. Indeed the facts of parthenogenesis 

 suggest that the originating stimulus is of secondary importance, for many 

 causes may set oif the chain reaction that is embryogenesis. The real 

 miracle is that unwanted embryos do not start more often; perhaps they 

 do as teratomas, those monstrous tumours composed of disorganized bits 

 of hair and cartilage, bone and epithelium. 



Space does not permit consideration of the many possible biological 

 roles of tissue PCO2 in tissue culture, cytostasis, neoplastic growth, sexual 

 maturation in both plants and animal tissues, limnology, etc. Some of these 

 aspects were discussed at the 17th Symposium of the Society for the Study 

 of Development and Growth [3]. Suffice it to say that work for many hands 

 exists within this difficult but rewarding area, work that can aim at repro- 

 ducing on the level of an isolated cell all the conditions found w^ithin a 

 developing tissue, so that such an isolated cell diff^erentiates morphologi- 

 cally just as if it were still surrounded by its normal cell neighbours. 



