ON DEVELOPMENT ETC. OF PHOKONI«. 025 



iuiiud a few that have already apposed themselves ilatty to the 

 ectoblast (see fig. 29 j, while the cavity behind the blastopore still 

 remains without a mesoblastic lining. This last condition persists 

 till the period when the nephridial invagination makes its appear- 

 ance. The state of things in question is to be seen in. lig. 20, 

 which represents a median sagittal section through an embryo (jf 

 nearlv the same staçre as i'ar. cS. 



Soon afterwards the anterior diverticula and the ventral 

 groove entirely disappear and the preoral lobe begins to bend more 

 distinctly downwards. Meanwhile an unpaired ectoblastic invagi- 

 nation appears at the posterior end of the larva, on the ventral 

 side of the blind end of the now greatly elongated gut. It appears 

 at first as a shallow depression (fig. 33, 7iep. p.) of purely ectoblastic 

 nature, having nothing to do with the mesentoblast. It is from 

 this invagination that the future nephridia of Actinotrocha de- 

 velop and hence I shall call it the nephridial pit, in preference 

 to the name " anal pit " of Caldwell, who tor the first time des- 

 cribed this structure. I have very frequently noticed signs of 

 vigorous cell-division in the cells of the pit wall, evidently only 

 for enlargement of the pit itself, since the axis of the karyokinetic 

 spindle is always placed paratangentially to the wall. I have 

 moreover often noticed peculiar ectoblastic cells round in shape and 

 in process of multiplication, situated just outside the edge of the 

 entrance to the pit (fig. 33). 



In larvte of the stage of fig. *) tlie ne])hridial pit can l)e well 

 seen in surface views. This stage further attracts our special at- 

 tention on account of several inq)ortant developmental processes 

 taking place in it. First to be noted is the fact that from the 

 posterior end of the primary gut a small and short evagination 

 protrudes itself touching the ectoblast with its blind posterior 



