CELLULAR BASIS OF MORPHOGENESLS IN THE SEA URCHIN 505 



regions has acquired certain mesenchymal properties as a result of its close 

 contact with the primary mesenchyme. One might denote this as a kind 

 of induction. 



The ectoderm is, however, not only a toy with which the mesenchyme 

 plays — it is to a great extent the ectoderm which guides the mesenchymal 

 pseudopodia and thus the morphogenesis of the entomesoderm. One may 

 thus say that the ectoderm serves as a kind of template for the entomeso- 

 derm, but of course not a template in the biochemical sense. 



How does the ectoderm guide the mesenchymal pseudopods ? The 

 films indicate that the ectoderm in some regions has a high "stickiness" 

 for the pseudopods, i.e. permits the pseudopods to attach strongly. The 



Fig. 6. A developing scheitel (dorsal extension of the ectoderm) in a young 

 pluteus stage. The extension of the ectoderm is brought about by the plug of 

 mesenchyme attached to the growing skeleton rod in the same way as in the 

 extending arms, cf. Fig. 4. 



Stickiness is lower in other regions. There is, in other words, a charac- 

 teristic pattern of stickiness at the inner surface of the ectoderm. But how 

 do the mesenchymal pseudopods find the areas of high stickiness ? 



The films show that the pseudopods of the primary mesenchyme are 

 very long and numerous and that they appear to explore the whole inner 

 surface of the ectoderm, cf. Fig. 7. During this apparently random ex- 

 ploration they come in contact with regions of low stickiness as well as 

 regions where the stickiness is high. In the latter case they attach to the 

 ectoderm and contract and thereby carry the cell body in the direction of 

 the sticky region. In the latter case the pseudopods may either collapse — 

 to be succeeded by new pseudopods — or continue their random exploration 

 until they reach a region where the stickiness of the ectoderm is high. The 

 primary mesenchyme as a whole will therefore gradually arrange itself 

 into a pattern which corresponds to the high points of stickiness of the 



