organisers: inducers of differentiation i8i 



location of the skeleton in the larva. This skeleton arises from groups 

 of primary mesenchyme cells, which are normally to be found on 

 either side of the blastopore at the close of invagination. If these 

 cells are scattered through the blastocoel by shaking, they return 

 to their original position.^ It would appear that the ectoderm near 

 the line of its junction with the endoderm exercises a specific 

 attraction on these mesenchyme cells, and this view is further sup- 

 ported by the following experiments. When sea-urchin larvae are 

 made to develop in water to which lithium salts have been added, 

 the proportion between the relative amounts of tissue devoted to 

 the formation of endoderm and ectoderm is altered, to the ad- 

 vantage of the endoderm and at the expense of the ectoderm, with 

 increasing concentrations^ (see p. 334). The ectoderm may be re- 

 duced to a tiny region occupying the animal pole, and in such 

 larvae, the skeleton-forming cells are to be found there, and not in 

 their normal position near the vegetative pole. 



The ectoderm is thus responsible for the localisation of the 

 skeleton-forming cells, and, in addition, it appears to control certain 

 details of the growth of the skeleton itself. The mesenchyme cells 

 secrete a triradiate spicule, apparently as an act of self-differentia- 

 tion. The type of the spicule is also a result of self-differentiation, 

 as is clearly seen in those experiments in which micromeres (pre- 

 sumptive skeletogenous mesenchyme cells) of Echinocyamus (which 

 normally possesses complex spicules) are grafted into the animal 

 half of a blastula (presumptive ectoderm) of Echinus (which normally 

 possesses simple spicules). The spicules ultimately developed in 

 such larvae are of the complex type.^ But the growth of the various 

 spicules and struts characteristic of the pluteus skeleton is depen- 

 dent on the ectoderm. This has been shown by experiments 

 similar to those described above, in which the relative proportions 

 of ectoderm and endoderm are varied. If the ectoderm is very 

 deficient, skeleton production goes no further than the triradiate 

 stage, in spite of the fact that the mesenchyme cells are present in 

 ample quantity. With increasing development of ectoderm, and 

 particularly of the ciliated band, there is progressive development 

 of the skeletal arms.* As we have already seen (p. 174), the pressure 



1 Driesch, 1896. - Herbst, 1895. 



^ von Ubisch, 193 1. * Runnstrom, 1929. 



