446 STUDIES ON THE EMBRYOLOGY OF THE SIPUNCULID^. 



These cells now degenerate; their yolk, and finally even their nuclei pass back- 

 ward into the newly formed ccelom (Fig. 15). The somatic plate or ectoderm of 

 the trunk meanwhile is growing forward and ventrad beneath the prototroch on each 

 side as in Sipunculus, dorso-lateral proliferations of the apical plate extend backward, 

 and thus the definitive body-wall of the larva is finally completed. The dissolution 

 of the prototroch cells in P. gouldii is effected as follows: The inner side of the cells 

 first shows signs of breaking down in that the cell-wall is dissolved, and the yolk- 

 granules pass inward and backward into the ccelom (Fig. 15). The outer parts of 

 the cells, however, remain intact for a considerable time and still contain nuclei. 

 When their dissolution is complete and their contents in the form of yolk-granules 

 have found their way backward into the body-cavity, the ectoderm of the trunk 

 closes over the gap left by the passage backward of the substance of the prototroch 

 cells, and becomes united to the apical plate laterally, as was previously the case 

 upon the dorsal and ventral sides (Fig. 16). 



I am of opinion that the mechanical pressure of the apical plate upon the 

 disintegrating prototroch cells during the periods of contraction of the retractor 

 muscles has an important part to play in crowding the remnants of the cells back 

 into the ccelom. 



The shedding of the zona radiata occurs simultaneously with the end of the 

 process of dissolution of the prototroch and of the engulfing of its substance into 

 the ccelom. The remnants of the zona radiata may be found still clinging to the 

 heads of embryos in which the remains of the prototroch cells have sunken away 

 from the surface. This appears to be a period fraught with considerable danger of 

 rupture of the lateral walls of the head region, and individuals are not infrequently 

 seen in which the substance of the prototroch has oozed out upon the surface of the 

 body through the premature tearing of the zona radiata, the cuticula in that region 

 being exceedingly thin. Hence it happens that the young larva, no longer a trocho- 

 phore, remains for a longer time than usual in a condition of contraction as regards 

 the retractor muscles until the prototroch region has healed over, so to speak, by 

 the growth of the ectoderm of the sides of the trunk forward to the apical plate and 

 over the region of the dissolving prototroch. 



