ASPLANCHNA EBBESBORNII (ROTIFER) 409 



97), which is due to the folds in the body wall. The pharynx 

 and stomach are now united by a distinct tube, the oesophagus 

 (figs. 97 and 98) and the digestive system is now completely 

 formed and differentiated into the following regions: buccal 

 pouch, pharynx, oesophagus, and stomach. The trophi or jaws 

 are formed from the ventroposterior wall of the pharynx (fig. 100). 

 The gastric glands are formed at the junction of the oesophagus 

 and the stomach (figs. 103 and 104). A few of the muscles which 

 are directly connected with the different organs are represented 

 in figures 98, 101, and 102. Figures 96 to 101 show the gradual 

 straightening of the embryo, and the migration of the mouth to 

 its definitive position. A small part of the foot still persists. 



The trochal disc with its cilia (fig. 99) is completely formed. 

 Its relation to the buccal pouch is represented in figure 100, a 

 horizontal section of the same stage. The invagination of the 

 ectoderm to form the urogenital sinus (figs. 98 and 101) corre- 

 sponds to the proctodaeum when an intestine is present. It is 

 formed on the dorsal side of the foot near its base, but when the 

 foot is completely absorbed the opening is considered as being 

 on the ventroposterior end. Figures 102 and 103 represent 

 horizontal sections of figure 101, taken near the ventral and dorsal 

 sides, respectively. The body cavity at this stage becomes very 

 prominent and the difi'erent organs within reach their definitive 

 condition. Figure 104 represents an embryo which has reached 

 its distinct adult condition, represented as a transparent object 

 from the dorsal side. All of the more important organs are 

 shown. The digestive tract has reached its complete develop- 

 ment as it occurs in the parthenogenetic female. The cells of 

 the ectoderm lose their boundaries and become a definite syncytial 

 layer, and the ectoderm with its delicate smooth cuticula con- 

 stitutes the body wall. 



The early stages in the development of the digestive tract of 

 the male and female are similar, but when the male embryo has 

 reached the condition represented in figures 79 and 87, there is 

 a temporary fusion of the cells of the wall of the pharynx with 

 those of the entoderm as in the female. The cells which, in the 

 female, produce the oesophagus, cease to divide in the male 



