314 WM. A. KEPNER AND FRANK HELVESTINE, JR. 



conspicuous vacuoles nor secretion products within the cyto- 

 plasm (fig. 9, A). In the older cells vacuoles appear within the 

 cytoplasm, become larger and more crowded with secretion prod- 

 ucts, until they occupy the greater part of their respective cells 

 (fig. 9, B). The cytoplasm of such cells is conspicuous only at 

 the fundus of the cell body. In this basal mass of cytoplasm is 

 the nucleus (fig. 9, B). It appears that only these large cells at 

 the periphery discharge their contents into the lumen of the 

 pharynx, for no empty vacuoles are found in the cells of inter- 

 mediate size. Many empty large cells occur at the periphery of 

 the pharyngeal wall (fig. 9, C). The staining reaction of the 

 empty cells is wholly basic, taking the acid dyes. Bordeaux red, 

 for example, or eosin stains all the details of these empty cells 

 red. This is in marked contrast to the reaction of the young 

 and secreting cells. In these the nuclei are highly acid in their 

 reaction, while the secretion granules show a great affinity for 

 the basic dyes. 



The origin of the pharynx is readily studied in dividing speci- 

 mens. Animals are not infrequently found which show two 

 planes of division, thus presenting a chain of three zooids, while 

 specimens with two zooids are very common at any season of 

 the year. 



The division of a Microstoma to form two zooids involves the 

 division of the enteron, the formation of a new 'brain,' new cili- 

 ated pits, gonads, and pharynx. We are concerned here only 

 with the origin of the pharynx; but it is an interesting fact that 

 the developing 'brain' of the posterior zooid remains in contact 

 along its anterior dorsal surface with the enteron until the phar- 

 ynx has become well established and even connected with the 

 enteron (figs. 10, 11, and 12, Br.). This 'brain' makes its appear- 

 ance earlier than does the anlage from which the pharynx is dif- 

 ferentiated. The pharynx arises as an invaginated sac of ecto- 

 dermal epithelium. At first the wall of the young pharynx is 

 composed of but a simple, ciliated columnar epithelium without 

 any glandular differentiation (fig. 11). Soon, however, a three- 

 fold differentiation of the cells of the ectodermal sac sets in. This 

 differentiation involves, a) a lining epithelium of the pharynx; 



