406 GEORGE W. TANNREUTHER 



little later stage than the preceding, from the right side, shows 

 the position of the central entodermal mass and its relation to the 

 embryo as a whole. The ectodermal cells at the anterior end 

 of the embryo now divide very rapidly and later contribute in 

 part to the formation of the stomodaeum and the pharjnix (figs. 

 73 to 75). The central mesentodermal mass of cells becomes 

 differentiated into two distinct regions (figs. 76 and 77), the 

 entoderm proper, which produces the stomach, oesophagus, and 

 digestive glands; and the part which produces the reproductive 

 organs with a few of its controlling muscle fibers. 



FORMATION OF THE TROCHAL DISC 



The developing embryo is divided into three distinct regions ; 

 the body, the head bearing the trochal disc, and the foot. The 

 foot is an embryonic structure and is absorbed before the birth 

 of the individual. 



The trochal disc begins as ectodermal prominences or growths, 

 due to a prohferation of cells on the ventro-anterior region. 

 Figure 73, viewed from the ventral side, shows a small lateral 

 fold on either side. A stage httle later than the preceding is 

 shown in figure 74 from the right side. These ectodermal folds 

 extend toward the posterior end, but only the anterior ends 

 contribute to the trochal disc. This figure shows the first steps 

 in the differentiation of the animal into three distinct regions 

 (head, body, and foot). The folds are more developed in figure 

 78, and the large central depression between the lateral folds is 

 open at either end. Later stages are represented in figures 81 

 and 83. The position of the mouth is visible in figure 81. The 

 embryo at this stage of development is considerably curved and 

 the ectodermal folds posterior to the stomodaeum have begun 

 their development. The extreme anterior folds later become the 

 dorsal part of the trochal disc. The position of the mouth is 

 now more definite. The lateral, anterior, and posterior folds are 

 now continuous (fig. 89). This figure shows the mouth close to 

 the posterior prominence, which, later, when the embryo 

 straightens out, becomes ventro-anterior. Figures 91 to 93 show 

 the foot at its maximum development, with its two pointed toes. 



