408 GEORGE W. TANNREUTHER 



The inner cell mass, as shown in figure 77, becomes differen- 

 tiated into the parts which produce the pharynx, the oesophagus, 

 stomach, and the gastric glands, and those which form the repro- 

 ductive structures. A later differentiation is indicated in figure 



79, where the lumen of the stomach is present and the pharjnrigeal 

 wall has become several layers thick on its posterior face. Figures 



80, 82 and 88 represent horizontal sections of the preceding 

 figure at different levels. The space which corresponds to the 

 body cavity is evident at different points. These three sections 

 as a whole show little differentiation and are rather hard to 

 interpret unless directly compared with the figure from which 

 they were taken. In the further growth and curvature of the 

 embryo the upper wall of the pharynx fuses with the entodermal 

 cells (figs. 85 to 87) forming the region which later is differen- 

 tiated into the oesophagus and the gastric glands. In a medium 

 longitudinal section of an embryo (fig. 87) corresponding to 

 figure 86, a large concavity is evident on the ventral side. This 

 is true of all embryos, when the invagination in the formation of 

 the pharynx has reached its maximum extent in a dorsoposterior 

 direction. Figure 89 (ventral view) represents the condition of 

 the embryo, when the pharynx has reached its maximum devel- 

 opment. The anterior and posterior ends of the embryo are in 

 immediate contact and the mouth is in the center of the embryo 

 when taken as a whole. An optical section of the same stage 

 (fig. 90) shows the position of the mesentodermal cells as they 

 are forced nearer the posterior end. This shifting is due to the 

 invagination of the ectoderm in the formation of the anterior 

 end of the enteric canal. The above figure shows the connection 

 of the pharyngeal and the entodermal ceUs. 



The embryo at this stage of development has reached its 

 maximum curvature and growth in length (figs. 91 to 93), but 

 does not resemble the adult rotifer. The embryo now begins to 

 straighten out (figs. 94 and 95), the foot is gradually absorbed, 

 and the mouth is carried more anteriorly (fig. 96) during the 

 process. The invaginated cavity is differentiated into the 

 buccal cavity and the pharynx proper. The embryo during its 

 period of maximum curvature has a segmented appearance (fig. 



