CLEAVAGE 63 



if there is a direct relation between cleavage pattern and embryo. He con- 

 cluded that any part of the egg, in the early cleavage stages, is capable 

 of forming a whole animal; and he spoke of such eggs as 'regulation' eggs, 

 all of whose parts are 'equi-potential' and capable of regulating so as to 

 replace any part that might be removed. The critical reader will notice 

 that Driesch drew conclusions about the behaviour of all parts from 

 experiments in which he had actually only succeeded in isolating certain 

 parts ; thus he always cut the egg parallel to the first cleavage planes, which 

 are vertical, and he was not justified in assuming that if he had cut it along 

 a horizontal plane, those halves would also regulate completely. As we 

 shall see later, in fact they do not do so ; and it is probable that no egg is 

 actually a completely regulation egg in Driesch's sense. 



Very soon after Driesch's discovery, similar experiments on other eggs 

 turned out in exactly the opposite way. Roux, for example, found that 

 when one of the first blastomeres of a frog's egg is killed, the other 

 develops into half an embryo; and the same thing occurred in ascidians 

 and many of the spirally cleaving eggs. Embryologists began to speak 

 of 'mosaic' eggs, contrasting these with the regulation type, and supposing 

 that they contained a mosaic of different cytoplasmic regions each of wliich 

 irrevocably developed into some specific part of the embryo. But in this 

 concept again they were going beyond their actual facts. The experi- 

 mental results showed that when certain eggs were injured, the remaining 

 parts did not regulate so as to compensate for the loss; but this does not 

 necessarily imply that all regulation is impossible in such eggs. We shall 

 see later that this is not only a logical non sequitur, but is controverted by 

 the facts which have become known more recently. In fact, there seems 

 to be no more a completely mosaic egg than a completely regulation one. 

 All eggs, we shall find, partake of both characters; all have some definite- 

 ness of localisation of parts with specific properties; and all have some 

 capacity for adjusting themselves to injuries. It is true that in some eggs 

 the localisation is the more striking phenomenon, in others the regulation; 

 and many eggs can still be looked on as tending to one or other extreme ; 

 but the differences are not absolute, and the two pure types do not exist. 



3. Differentiation without cleavage 



In the early years of the century F. R. Lillie discovered that partheno- 

 genetically activated eggs of the polycheate Chaetopterus may sometimes 

 achieve a considerable degree of differentiation although remaining in an 

 undivided state. In these eggs the cleavages are, however, not totally 

 suppressed. Pasteels (1934) and Brachet (1937), who have re-studied the 

 material more recently, point out that the activated egg becomes lobulated 



