504 T. GUSTAFSON 



restricted capability for strong deformations. It may change its shape some- 

 what (e.g. form a ciHary plate and ciliary bands, undergo a dorso-ventral 

 flattening and form arm buds) as a result of a change in adhesion between 

 some of its cells. When it deforms strongly, the forces required are pro- 

 vided by the mesenchymal and mesodermal elements : the extension of the 

 arms, Fig. 5, and the scheitel (the dorsal extension of the ectoderm), Fig. 6, 

 are thus dependent upon the clusters of skeleton forming mesenchyme 

 cells which collect at the tips of the skeleton spicules and push the ecto- 

 derm forwards. As a further example, the dilatation of the mouth appears 



a 



mmh 



Fig. 5. Anal arms (seen from the anal side) in larvae of Psamnucliiinis miliaris 

 in early pluteus stages, a, arm with the typical cluster of mesenchyme attached 

 to and forming the growing skeleton. The extension of the arm rudiment is 

 brought about by the pressure of the mesenchyme cluster. The region between the 

 two arms (right arm outside the picture) shows a pulsatory activity which brings 

 about an invagination, b, the pulsatory activity between the anal arms is supple- 

 mented by the emission of contractile pseudopodia which, for example, attach to 

 the skeleton and exert a tension which partly causes a release of the pseudopod- 

 forming ectoderm cells. 



to depend upon the contractility of the oesophagus. (The expansion of the 

 stomach rudiment into a thin-walled vesicle is also greatly dependent upon 

 a contractility outside the rudiment itself, i.e. is brought about by the 

 hydrostatic pressure generated by the contractions of the oesophagus.) 

 The ectoderm may, however, acquire a pulsatory and pseudopodal activity 

 in certain regions, i.e. in the regions on the ventral side which early have 

 been in close contact with the ventral clusters of primary mesenchyme. I 

 refer to the invaginating regions between the arm rudiments. The in- 

 vaginations are brought about by pulsatory and pseudopodal activity. 

 Fig. 5, and thus are reminiscent of the invagination of the archenteron 

 rudiment. It may be permitted to suggest that the ectoderm in these 



