No. I.] THE EMBRYOLOGY OF CREPIDULA. 157 



of them, E^, is shown dividing, and this is the only case of divi- 

 sion among them which I have ever seen ; in later stages of 

 other eggs, e.g.. Fig. 65, they are still undivided. In Fig. 68 

 four nuclei are shown beneath the ectoderm and posterior to 

 the cell 5D. They lie at the extreme posterior end of the 

 archenteric cavity, and from their position and grouping I 

 believe they are the nuclei of the enteroblasts. 



In Figs. 63 and 65 the anterior enteroblasts, E' and E^, are 

 shown displaced to the right (left in the figure, which is seen 

 from the ventral side). This displacement is probably associ- 

 ated with the rotation of 5D and the laeotropic torsion of all 

 parts posterior to the foot. 



5. Organs formed from the Entomeres. 



(a) TJie Archenteroit. — The four macromeres form the roof 

 of the archenteric cavity ; the cells of the fifth quartette form 

 its lateral boundaries, inclosing the cavity on all sides save the 

 posterior. Here the archenteric cavity runs backward between 

 the cells 5C and 5D, nearly to the posterior boundary of the 

 &gg, Figs. 57, 60, 6Z. The cells of the fourth quartette come 

 together on the ventral side of the archenteron, forming its 

 floor anteriorly, and ultimately giving rise to some of the many 

 small cells which form that part of the mesenteron, adjoining 

 the stomodaeum, Figs. 90, 93, 95. Thus the whole mesen- 

 teron becomes surrounded with yolk cells, all of which have 

 their nuclei next the cavity of the mesenteron. 



(b) The Intestine. — In the latest stages in which they can 

 be recognized the four enteroblasts form the ventral wall of 

 the posterior prolongation of the mesenteron, Figs. 60-68 ; 

 and since this prolongation ends near the middle of these four 

 cells, they may also be said to form its posterior wall. This 

 hollow process from the mesenteron is the fundament of the 

 intestine, as further development plainly shows, and it is there- 

 fore almost certain that the enteroblasts form a portion of the 

 walls of this organ. 



The relation of these cells to the fundament of the intestine 

 is still more plainly shown in actual sections of these stages. 



