CELLULAR BASIS OF MORPHOGENESIS IN THE SEA URCHIN 5OI 



primary mesenchyme into the blastula cavity is an example of this activity, 

 cf. Fig. I and [i, 6]. If the celkilar adhesiveness remains unchanged, on 

 the other hand, the pulsatory activity and their change in shape bring 

 about an invagination of the body wall. The first phase of invagination of 

 the archenteron and the onset of the evagination of the coelomic sacs from 

 the archenteron tip are examples of such a process, cf. Figs, i, 2 and [3, 6]. 

 B. The pseudopod activity brings about a strong extension of the 

 invaginated or evaginated regions of the body wall. The second phase of 

 invagination, cf. Figs, i, 2 and [i, 3], and the extension of the coelomic sac 

 rudiments [3] are brought about by such a mechanism. The bending of 



8 hours 



Fi<i. 3. Diagram of the course of invagination of the archenteron in a larva of 

 Psdnimechiiius miliaris (the larva in Fig. i). Inner height of the archenteron in 

 microns is plotted against relative age in hours, the first stage arbitrarily called 

 o hr. The pseudopodal activity is symbolized by the horizontal lines below the 

 curve to the right, each line representing one visible pseudopodium ; dashed line : 

 pseudopodium intermittently visible ; wavy line indicates direct contact between 

 the secondary mesenchyme and the ectoderm. The schematic drawing to the left 

 gives the appearance of the larva in three diflFerent stages corresponding to the 

 marks below the abscissa. 



the archenteron tip towards the presumptive stomodaeum region of the 

 ectoderm (Fig. 4 and [2]) is another result of pseudopod contractions. The 

 contractions of the pseudopods will finally pull the pseudopodia-forming 

 cells out from the archenteron tip and a so-called secondary mesenchyme 

 is thus formed. The pseudopod activity finally brings about a rapid 

 migration of the liberated primary as well as secondary mesenchyme cells. 



C. The role of a decrease in adhesion between the cells in the wall in 

 the larva has already been exemplified. A strong increase in adhesion 

 between the cells may cause them to increase their contact surfaces so that 

 they become more or less cylindrical. This appears to be the basis for the 

 formation of the ciliary bands and the ciliary plate in the animal pole. 



The activities mentioned appear to be released according to a simple 



