MKM01K3 OF THE NATIONAL ACADEMY OF SCIENCES. 



415 



The two divisions of tbo foregut, oesophagus, and masticatory stomach, have tin- relations 

 already described. The wall consists of a single layer of tall columnar cells. In the masticatory 

 division the wall has begun to thicken next the yolk screen, its cells being cuneiform, and the 

 nuclei elongated and crowded below the surface. From the anterior wall, muscle or connective 

 tissue cells extend forward under the brain. 



HG 



FIG. 1. Diagrams of transverse sections through the alimentary tract of an embryo of Alplicux aaulcyi u liich is 

 nearly ready to hatch, to show tin- origin of I In- gastric uland It tun tlie postero lateral lobes of the in id gut. Si (tion 

 I cats tbo hindgut and lobes of the "liver," Section ill the. hinduut. \\-hero it merges into the mcsenti ion. : i : i'. ;/;/% 



Secoudai y lobules of tii'J ' . UO, li indent ; nKj 1 , posti i o lateral lol" s ol inidgllt. 



The development of the meseuteron can be understood by reference to Figs. 162-165, 168, and 

 185. The endodermal epithelium spreads by the division of its own cells and by accession of cells 

 from the yolk, both forward over the nervous system and upward against the sides of the body. 

 This is shown in the series of horizontal sections (Fig. 162-165). Fig. 168 which is from an 

 embryo a little more advanced, shows that the endoderm is rising from the nervous cord near its 

 point of flexure, into ti transverse vertical fold. Simultaneously with the upward growth of this 

 ventral fold, two dorsal longitudinal folds grow downward, and finally unite with the ventral fold 

 and with each other, thus constricting oil' from the alimentary tract two lateral pouches, the pri 

 irary lobes of the " liver." The folds grow forward and the constriction proceeds gradually with 

 the growth of the embryo. This process is illustrated by the diagrams (Fig. 1) which were drawn 

 from tin embryo near the point of hatching. The histology of the endoderm as shown in Fig. 173, 

 is essentially the same as in the previous stage. The cells are prismatic, and the. undent spherical, 

 and, as in all stages, filled with numerous nucleoli or chromatiu balls. The cell walls are very 

 delicate ?.ud the protoplasm often contains large vacuoles. 



US. 



I''n:. 2. .Semidia:;raiiin>atic representation of tho alimentary tract and it.-, :i|>poudii};e in the 

 rilM larva ol .1 ljili< '"-v .s-rtf'/'V/i. Tin- middle 1 lie- nl' the body is also sliinyn. /-'N, I'ure^ilt ; (J(J 1-3, 

 secondary lobules of iiostcrolatei.il lobu of midmil ; lift, hind-ill /;/, mid^ilt ; 1113 1-3, an- 

 terior, lateral, and postero- lateral divisions of niidgut; mo, mouth. 



