XVI ECHINODERMATA - 483 



when dealing with the Echinoidea, but there are some which are, 

 so to speak, peculiar to the Asteroidea. 



It has been already mentioned that the fully developed gastrula of 

 Asterias is a long, sausage-shaped structure, and that the archenteron 

 only reaches half-way through it (Fig. 356). Driesch (1895) made a 

 thick culture of these gastrulae, accumulating hundreds in a very 

 small quantity of water, and snipped at random in this water with 

 a fine pair of scissors about 200 times. In this way he succeeded 

 in cutting a number of the gastrulae in pieces. Sometimes he found 

 that they were cut longitudinally and sometimes transversely. In 

 both cases, if the fragment included both endoderm and ectoderm, it 

 healed up by the approximation of its edges and formed a miniature 

 gastrula, which then developed further into a small but perfect 

 Bipinnaria. If, however, the gastrula was transversely bisected after 

 the thin-walled vesicle at the apex of the archenteron had made its 

 appearance, so that this vesicle was removed, then the truncated larva 

 healed up and went on developing, but it never formed a new 

 coelomic vesicle, although it takes on externally the form of a 

 Bipinnaria. The appearance of this vesicle, therefore, according to 

 Driesch, "negatively determines," i.e. limits the potency, that is, the 

 power of development possessed by the archenteric wall. Before 

 that vesicle appears a small fragment of this wall will grow and 

 rearrange its cells so as to form the three segments of the asteroid gut, 

 viz. oesophagus, stomach, and intestine, and in addition the terminal 

 vesicle; but, when this has been once formed, then, even a larger 

 fragment of the archenteron is incapable of moulding itself into 

 more than oesophagus, stomach, and intestine. 



Driesch regards this limitation of power as due to a progressive 

 " stiffening " of the protoplasm, which renders it less and less amen- 

 able to the regulating influence of the " entelechy," or indwelling 

 power, which, according to the vitalistic principle held by Driesch, 

 knoivs and wills what it wants to do with the material at its disposal. 

 A more humble explanation, and one more in accord with what we 

 know of other eggs, is that there exists a definite coelom-forming 

 substance which is at first diffused through all the cells of the gut 

 wall, but which becomes, as development proceeds, definitely localized 

 in one spot ; and if that portion of the gut be removed, the remainder 

 has no material which will allow of the development of the coelom. 



Herbst (1896) showed that if a solution of 3-7 per cent of sulpho- 

 cyanide of potassium be made, and then three parts of this solution 

 be added to 100 parts of sea- water, the eggs of Asterias will develop 

 in this medium and will live on for four weeks, but that they do 

 not get beyond the blastula stage. The beginning of an archenteron 

 may be formed, but it degenerates into a granular mass of cells and 

 is absorbed. 



These persistent blastulae, though possessing the clear trans- 

 parence which indicates health, differ from the normal blastulae in 

 possessing abundance of mesenchyme in the interior. The normal 



