ASPLANCHNA EBBESBORNII (rOTIFER) 407 



It projects forward over the anterior end of the embryo and 

 obscures a part of the fold which contributes to the formation of 

 the disc. These figures show how the disc is depressed posteri- 

 orly on the ventral side. The dorsal part of the disc is indicated 

 in figure 92. The entire disc in its final stage of development 

 with the outer row of cilia, is represented in figure 99, ventral 

 view. The trochal disc is very retractile, and its activities are 

 regulated by a number of well-developed muscle fibers. The 

 cilia extend into the buccal pouch, but not into the pharynx. 

 The ciliary wreathes serve for locomotion and feeding. 



DIGESTIVE SYSTEM 



The major portion of the digestive system is derived from the 

 large mesentoblast cell E, the stomodaeum and phar3^nx from 

 the ectoderm. The early stages in the formation of the anterior 

 end of the digestive tract begin as an invagination on the ventro- 

 anterior end (figs. 71 to 76). The development of the head region 

 in the formation of the trochal disc is closely associated with the 

 origin and growth of the anterior end of the digestive tract, and 

 the prominence of the trochal folds is accentuated by the 

 stomodeal invagination. Figures 76 and 77 represent optical 

 sections from transparent whole mounts in the early stages of 

 differentiation in the region from which the pharynx develops. 

 The embryos are folded ventrally, making the dorsal side appear 

 abnormally long. These figures show that two distinct regions 

 are recognizable — an outer epithelial portion, which gives rise to 

 the definitive ectoderm and the musculature, and an inner 

 slightly differentiated region, which produces part of the enteric 

 canal and the entire reproductive system with a few muscle 

 fibers directly connected with it. As the cells invaginate to form 

 the pharynx, the products of the mesentoblast are forced more 

 and more posteriorly (figs. 75 to 77). The cells on the posterior 

 face of the invaginated cavity become several layers thick and 

 form the ventral and posterior walls of the pharynx, from which 

 the jaws are later developed. The anterior and dorsal walls of 

 the pharynx seldom become more than one layer of cells in thick- 

 ness. Rudimentary salivary glands are often present in connec- 

 tion with the walls of the pharynx (fig. 102). 



