518 STANLEY C. BALL 



In the description of the ectoderm formation there was pointed 

 out a pair of cells which, although at that time lying at the 

 anterior surface of the embryonic mass, did not enter into the 

 development of the body covering. These two cells, well shown 

 in figure 23, together with two or three others not seen in this 

 section proliferate the rod of cells which, as in Mesostomum, 

 grows inward to penetrate the mass which lies beneath the brain. 



An intimate connection between these cells and those of the 

 ectoderm is evident in figures 34 and 26. The former represents 

 a section which is obUquely sagittal. It is evident that before 

 the cells of the pharyngeal bulb have become closely grouped, 

 the posterior cells of the epithelial rod have penetrated the mass 

 of dark nuclei and lie against the entoderm. A distinctly clearer 

 cytoplasm and paler nuclei serve to distinguish them from the 

 neighboring cells. Each of their nuclei, like those of the ecto- 

 derm, with whose early history these cells are closely associated, 

 contains a nucleolus. Externally a slight invagination of the 

 ectoderm indicates the future position of the mouth. 



A slightly later stage is shown in figure 27, a transverse section 

 through the posterior lobes of the brain and the center of the 

 pharynx. Portions of seven clear cells belonging to the epithelial 

 rod are conspicuous in the center of the mass of nuclei which 

 have now become closely grouped with their cytoplasm in what 

 appears to be a syncytium. For a longitudinal view of the organ 

 at this stage the obliquely frontal section in figure 26 is of interest. 

 The nuclei of the bulb are disposed roughly in two rows, some- 

 what toward the periphery of the mass. As yet no cell mem- 

 branes appear between them. The epithelial cells are constricted 

 within the bulb so that they become elongated, parallel with the 

 longitudinal axis of the organ; one cell appears to traverse its 

 entire length. Those between the pharynx and epidermis are 

 irregularly arranged, while at the inner end of the pharynx is 

 a group of five similar cells. Three of these appear to be inti- 

 mately connected with the epithelial rod; the posterior pair, 

 however, are not thus closely associated, but have, it is believed, 

 an entodermic rather than an ectodermic origin. Further 



