5o6 T. GUSTAFSON 



ectoderm — the "template" [5]. The attachment of the cells to the ecto- 

 derm do not appear to be permanent, however. Preliminary observations 

 indicate that the contacts have a rather restricted life time, and the mesen- 

 chyme cells may thus change their distribution as a response to a further 

 elaboration of the pattern of stickiness of the ectoderm. I may finally 

 mention that the adhesion between the primary mesenchyme and the 

 ectoderm may be determined by the same factor that determines the 

 adhesion between the cells within the ectoderm : The primary mesenchyme 

 cells thus seem to accumulate in those regions where the ectoderm cells 



Fig. 7. Primary mesenchyme cells with exploring thin pseudopods (filopods). 

 Drawing from a time-lapse film of a young gastrula. The mesenchyme cells have 

 already arranged themselves into a ring (part of which is seen in the Figure) but 

 still explore the ectoderm in regions far outside the ring level. 



will come close together (become more or less cylindrical), e.g. to form 

 ciliated bands. The phenomenon of random exploration is also applicable 

 to the pseudopods formed by the secondary mesenchyme and the coelomic 

 sacs. 



As a general conclusion of this brief review we may state that the 

 morphogenesis of the sea urchin larva, in spite of its relative complexity, 

 is not completely obscure but appears to be resolvable into a restricted 

 number of morphological cellular activities, which appear to be released 

 according to a simple time-space pattern, i.e. as a wave proceeding from 

 the lower (vegetal) pole towards the higher (animal) pole. The activities in 

 the different zones bear a certain relationship to each other, i.e. the 



