CELLULAR BASIS OF MORPHOGENESIS IN THE SEA URCHIN 503 



in their mutual adhesion and their adhesion to the hyahne membrane. 

 These differences might be summarized as differences in the "strength of 

 their mesenchymal properties" of the cells in the different zones (a 

 provisional and perhaps vague and misleading concept). This intensity 



is highest in zone i : The cells pulsate and lose their adhesion for 

 each other and for the hyaline membrane before the pseudopodal 

 activity starts. 



It is weaker in zone 2 : The cells pulsate but their adhesive proper- 

 ties onlv gradually decrease and the cells are only released as a 

 result of a strong pull of their pseudopods. 



It is weaker in zone 3 : The coelom cells show some pulsatory 

 activitv and form pseudopods but they never pull themselves out 

 but remain connected to each other to form cell sheets, the walls of 

 the coelomic sacs. 



It is weaker in zone 4 : The oesophagus cells pulsate somewhat, but 

 never emit pseudopods. There are, however, contractile elements 

 within the wall of the oesophagus, bringing about its periodic 

 contraction. One can imagine that these elements are, in a sense, 

 equivalent to contractile pseudopods. The main difference may be 

 that thev never shoot out from the wall. 



It is still weaker in zone 5 : The stomach cells may occasionally 

 pulsate but no contractile elements are present and the cells remain 

 connected to a sheet, the wall of the stomach. 



This graded change in properties appears to continue within the 

 ectoderm. The cells of this germ layer do not pulsate (with the exceptions 

 mentioned earlier) but the adhesion between the cells is different in 

 different regions. The cells in the thin epithelial sheets can be assumed to 

 have a comparativelv low adhesion for each other. In some zones, however, 

 the mutual adhesion increases, and the cells therefore increase their mutual 

 contact surfaces and become cylindrical or more or less hexagonal. This 

 occurs in the most animal region, the animal plate, and in the ciliary band 

 which extends from it. (This rearrangement of the cells is no doubt 

 responsible for the ventral flattening of the gastrula.) 



If this interpretation is correct, there is thus a more or less continuous 

 spectrum of morphogenetic properties along the animal-vegetal axis of 

 the larva. The closelv packed ciliary cells in the animal plate and the 

 ciliarv bands represent one end of this spectrum, the primary mesenchyme 

 cells derived from the zone at the vegetal pole represents another extreme 

 case. It is tempting to relate this spectrum in cellular activities to the 

 animal-vegetal gradients, so familiar to the embryologists, cf. [7]. 



The film and this review suggest that the ectoderm has a rather 



