220 WILLIAM BATE SON. 



occupying most of the space enclosed by the epiblast. Still 

 further backward the cavities in these two mesoblastic tracts 

 close up, and their walls are continued for a short distance as 

 two solid cords of cells^ and then disappear. 



The mesoblast of the anterior body cavity is, therefore, formed 

 directly from the walls of the hypoblast, which occupied the 

 same situation. It is separated off from it by a process of 

 constriction in the region of the external groove, dividing the 

 proboscis from the collar (fig. 13,^). While this process of con- 

 striction is being carried out, the pouch of mesoblast grows back- 

 wards, surrounding the gut except on the ventral surface, but 

 especially forming the hollow horns (fig. 28, r. M' ., I. M'.), 

 lying in a horizontal position, one on each side of the gut. 



These relations are made clear by the diagram (fig. 40), and 

 the continuity of the gut with this anterior mesoblastic wall is 

 shown best in longitudinal, median, vertical section (fig. 35. 

 This section is taken through a larva slightly younger than 

 Stage E.) 



The anterior body cavity is not completely constricted off 

 from the gut until a later period. 



In this larva the hypoblast itself is composed of large cells 

 with rounded outlines, containing elongated nuclei of variable 

 shape. Some of these cells are granular, while others have the 

 fluid appearance described above. In the regions which are 

 not concerned in the formation of the mesoblast, these cells lie 

 in two to four irregular layers. A section through the region 

 of the posterior horns of the walls of the anterior body cavity 

 is shown in fig. 28. It is in about this position that the 

 mouth is eventually formed on the ventral surface. The meso- 

 blastic tissue of the anterior body cavity is seen as two len- 

 ticular masses of large cells, lying on each side of the archen- 

 teron in the dorso-lateral regions of the body, their cavities 

 being closed. 



The Nervous System. — In the dorsal middle line of this 

 region a slight thickening of the epiblast is visible which is the 

 rudiment of of the central nervous system. As yet, however, 

 no further development of it is present (fig. 28), and it merely 



