Stxidies on the Development of Larvai Nephridia. 495 



of the closed primitive streak, as meutioned above. I do not think 

 tluit tbis structure is a true sac, in many larvae it is very imperfect, 

 in some larvae belüg- represented by a few scattered cells^. It 

 cannot be compared for a moment to the sac that later gives rise 

 to the large trunk coelom. It is perhaps almost unneeessary for 

 me to add that it is not a diverticulum of the gut, belng formed 

 strictly by the irregulär proliferation of cells from the anterior wall 

 of the archenteroQ. The body walls of the larva at this stage 

 become much thicker, and there is a rapid inerease in size. The 

 terminal portion of the primitive streak ends at the pointed end of 

 the larva in a slight depression which is the "anal pit" (fig. 27). 



On the external surface the primitive streak soon disappears 

 from the rapid growth of the ectoblastic elements. The anal pit 

 alone remains to mark its former posterior point. In trans verse 

 sections of the larva through the blastopore one finds a slight de- 

 pression just inside the blastopore on either side. These I take to 

 represent the anterior divertieula of Caldwell. They do not seem 

 to develop beyond the stage of very shallow depressions, and uever 

 give rise to divertieula as in the Japanese speeies. I cannot find 

 that they take an}' part in the formation of mesoderm. In the Faro 

 larva, they reach about the stage represented in Caldwell's (4) 

 fig. 8, after which they disappear. Soon the pre-orai lobe bends 

 down ventrally, as shown in figs. 8, 9 and 26, the anal pit at the 

 posterior end of the larva becoming deeper and more marked. I 

 eonsider the anal pit to mark the former terminal point of the 

 primitive streak, although this has been much debated, the 

 actual pit itself is a later development. The anal pit therefore cannot 

 strictly be called a portion of the blastopore, but is a growth from 

 the ectodermic surface, it rapidly grows at the expense of the 

 ectodermic cells which sink in as the pit forms. After this stage 

 it is best to call it by the more suitable uame of nephridial pit, 

 as the nephridia subsequently arise in connection with it. I think 

 the whole growth and closure of the blastopore in Phoronis jioint 

 to this opening as being the original opening of both mouth and anus. 

 The close relation of the anus to the terminal portion of the primitive 

 streak points to its formerly being included within this structure as 

 a part of the blastopore. 



The formation of mesoderm in Phoronis is a general 

 process in which the whole endoderm takes more or less part. In 



1 See page 505 further discussion. 



