464 • HAROLD HEATH 



specimens the blastocele contains a ground substance distinctly- 

 stained by Delafield's haematoxylin, and is to that extent in 

 contrast with the fluid contained in the vesicle, which has only a 

 slight affinity for dyes of this character. 



After cells of the anterior vesicle have separated and migrated 

 into the blastocele, many of the mesenchyme elements from the 

 apical plate move to the wall of the gut, especially in the region 

 of the oesophagus. In the oldest larvae in my possession (about 

 twenty hours later than the stage represented in figure 2) a few 

 of these apical mesenchyme cells, more or less bipolar in form, 

 rest upon the wall of the anterior half of the oesophagus at right 

 angles to its long axis. Between this point and the stomach 

 other mesenchyme cells of entodermal origin are taking up similar 

 positions. Both types of mesenchyme doubtless become trans- 

 formed into circular muscles. 



The left hydrocele, after separating from the archenteron, is 

 not only larger than the right, but its .subsequent growth is more 

 rapid, and it soon develops a pore canal and hydropore which 

 have no counterparts on the opposite side; at least none have 

 been discovered in the material at hand. 



A posterior enterocele pouch (fig. 4) arises from the left side 

 of the gut at about the middle of the future stomach. It is a 

 hollow outgrowth, nearly as thick-walled as the gut itself, and 

 after severing its connection with the digestive tract, it be- 

 comes a flattened vesicle wedged in between the stomach and 

 body wall. In a stage slightly older than the one represented in 

 figure 2 the left hydrocele forms a diverticulum at its posterior 

 end which ultimately comes in contact with the posterior en- 

 terocele pouch and fuses with it. Before this fusion takes place 

 the posterior enterocele vesicle, in several larvae, assumes the 

 form of a rather narrow ellipse with its long axis extending 

 from a point slightly behind the left hydrocele to the base of the 

 intestine. Nothing is known regarding its subsequent history. 



Comparisons. Since the year 1869, when Metschnikoff first 

 called attention to the resemblance between the echinoderms and 

 Balanoglossus a vast amount of data has accumulated. This 

 tends to support the original theory that the two groups in ques- 



