No. I.] GERM-LAYERS TN CLEPS/NE. 137 



larger cells, destined to form the several pairs of massive cell- 

 groups, known as the salivary glands (s^/), which are later 

 found on the dorsal and ventral sides of the pharynx, with canals 

 leading into the extreme hind end of the proboscis. The 

 epithelial portion extends through the proboscis, now distinctly 

 marked ofif from the rest of the stomodaeum, and reaches back- 

 ward between the dorsal and ventral masses of glandular cells. 

 There is still no lumen recognizable in any portion of the 

 oesophagus, but tiie entodermal epithelium is so well differenti- 

 ated in color and general appearance, and agrees so perfectly 

 with the conditions seen later in the posterior portions of the 

 alimentary canal, that there is not the least difificulty in distin- 

 guishing it from the other embryonic tissues associated with it. 



On the dorsal side of the yolk the entoderm is still represented 

 by entoplasts {c;ip), but on the ventral side, by an extremely 

 thin layer of epithelial cells (en), which would be easily over- 

 looked, except for the strongly stained oval nuclei. In my study 

 of C. marginata I failed to recognize this very obscure layer, 

 and thus overlooked a stage of development which connects the 

 entoplasts with the epithelium derived from them. The same 

 condition is seen along the middle region of the ventral side, as 

 shown in Fig. 26, en, but the layer vanishes a little farther on, 

 behind which point we find scattered entoplasts (Fig. 27, enp). 

 The gradual transition from this flattened epithelium into 

 columnar epithelium is well shown in Fig. 25, which represents 

 the dorsal half of one of the anterior caeca in C. marginata, 

 nine days after hatching. 



Recapitulation. — The history of the mesenteron may now 

 be recapitulated. 



1. The earlier entoderm cells arise beneath the cephalic lobe, 

 and are probably budded off from the entoblasts, a, b, c, as 

 distinct cells, precisely as in Nephelis. But to these earlier and 

 regularly formed cells are soon added others, which appear 

 first as entoplasts, so that it is impossible to draw any line of 

 distinction based on the mode of origin. 



2. The larger portion of the mesenteron, embracing the 

 whole ahmentary tract, with the exception of a small, anterior 

 ((Esophageal) portion, passes through the following stages of 

 development. The first stage is represented by the three large 

 macromeres, or entoblasts {a, b, e) ; the second by ejitoplasts 



