(Ill) 



vatioi. i-or P. architecta. This ridge aoe?? .,ot seen, to l-.avc 

 any rudimeiit in the Ac ti nor, roc ha and it is not present just 

 after metairiorphosis. 



_ Suinraa .ry_>_ 



The male and feruale reproductive organs do not develop at 

 the same tirrie in P. architecta aiid the inaications are that it 

 is a protandrous animal, 



i^'ertilization in external and the eggs are not held in loph- 

 ophoral masses by tne tentacles. 



Segmentation is holoblastic and equal but cleavage does not 

 occur sinultaneously in all the blastomeres. During the divi- 

 sion of the four-cell stage into the elgtit-cell stage, the upper 

 four blastomeres rotate in tne direction of the hands of a watch. 

 The sixteen-cell stage arises from the eight- cell stage by a 

 meridional division o^ each blasLomere, 



The blastopore is eccentric from tiie beginning of gastru- 

 lation and the gaiiglion of the Actinotrocha makes its appearance 

 at thds time. As developmenc proceeds, the blastopore gradu- 

 ally closes up from Ihe posterior end toward the anterior end 

 of the larva until finally it becomes a transverse slit. 



The "primitive streak" of Caldv/ell does not seem to be pres- 

 ent in the larva of P. architecta. 



