ECHINODERMS 83 



the later developmental history of a blastomere can be altered if the cell 

 finds itself in an abnormal situation, as it does when isolated. This con- 

 clusion still stands. The cleavage pattern can be altered by various treat- 

 ments, and in spite of this, normal larvae are formed. Clearly the 

 pattern of cleavage has no decisive effect on the pattern of develop- 

 ment. 



Driesch went further in his conclusions and considered that every part 

 of the early egg had the same potentialities. He argued that the formation 

 of a complicated embryo from an egg all of whose parts were alike was an 

 inconceivable achievement to be accomplished by a natural mechanism, 

 and that its explanation demanded a non-natural agent, whose functions 

 were to create order out of uniformity; this he called 'the entelechy'. 

 But as a matter of fact, Driesch' s basic postulate, that all parts of the egg 

 are similar, has turned out to be untrue; and the entelechy thereby loses 

 its main support. 



The demonstration of differences within the egg has come from experi- 

 ments in which the parts have been cut apart along horizonal planes. 

 There is no need here to attempt to summarise the whole massive volume 

 of evidence, and we will select one of the most demonstrative experi- 

 ments — that in which the 32- or 64-cell stage has been dissected. Horstad- 

 ius, who did the experiment, distinguished five zones. Anifiicil-i and 

 animal-2 lie at the top, and below them are vegetative-i and vegetative-2 

 derived from the macromeres ; at the very bottom of the egg there are 

 the micromeres. In the first part of the experiment, each of these zones 

 was isolated. The essential feature of the result was that each zone devel- 

 oped more or less in accordance with its normal fate within the embryo, 

 only more so, if one may put it like that. For instance, an-i gave a pluteus 

 which contained only the organs appropriate to the most animal region 

 (e.g. apical tuft, no gut) ; but these organs were exaggerated, in the sense 

 that the apical tuft spread out to cover nearly the whole surface. An-2 also 

 gave larvae with no gut, and usually with apical tufts, which were often 

 enlarged (although normally an-2 does not participate in the formation 

 of the tuft, which is formed from an-i cells). Similarly the vegetative cells 

 gave larvae containing vegetative organs, which again inight be exagger- 

 ated. The micromeres unfortunately fail to develop anything when isol- 

 lated, but veg-2 gives a larva with an exaggerated gut, which is often too 

 large to fit inside the ectoderm and thus protrudes, the resulting embryo 

 being known as an exogastrula. Veg-i is highly variable; sometimes it 

 has exaggerated vegetative organs (e.g. gut), sometimes it gives a larva 

 consisting mainly of animal organs; in a few cases it produces a reasonably 

 normal pluteus. 



