140 BULLETIN OF THE 



tine. In the next stage shown (Plate VIII. Fig. 73), further changes are 

 seen to have taken place. The most prominent is the down-folding of 

 the lower wall of the intestine at its middle region to form the coecum. 

 Even at this early stage histological ditferentiation of the cells of this 

 region has occurred to such an extent that the lumen of the coecum is 

 nearly obliterated by the great elongation of some of the cells lining it. 

 This condition of affairs will be understood by studying the cross sec- 

 tion of the coecum at a later stage, as shown in Figure 94, Plate X. 

 The cavity of the rectum has also enlarged, and its cells have taken 

 on the regular columnar appearance which exists in the adult. 



At a still later stage (Plate IX. Fig. 77), the position of the cardiac 

 and pyloric valves, separating respectively the oesophagus (ce.) from the 

 stomach (ga.), and the coecum (cce.) from the rectum (rt.), is clearly in- 

 dicated. The blind sac is still further elongated and well differentiated 

 from both stomach and rectum. In order to attain the adult condition 

 (Plate VIII. Fig. 72), the oral portion of the alimentary tract has merely 

 to become divided, by a difference in the character of its cells, into 

 pharynx {phx.) and oesophagus (as.), the stomach (ga.) to increase in 

 diameter, and the blind sac (cce.) to elongate. The anus (an.) finally 

 comes to lie at the apex of a small cone, or sphincter valve. 



The histological changes which the cells of the different parts of the 

 alimentary tract undergo are considerable, and will be treated of in 

 order, beginning with the 



(Esophagus. — At a stage a little later than Figure 77, the oesophagus, 

 as is shown in Figure 84, Plate X., has a small diameter relative to that 

 of the rest of the alimentary tract (cf. Plate VIII. Fig. 72, ce.), and its 

 inner lining is composed of high columnar epithelium, like that of the 

 oral groove. The shape of the cells is not greatly different in the adult ; 

 but they become vacuolated, and since these vacuoles lie near the* base 

 of the cells, and either nearer to or farther from the lumen than the 

 nuclei, the latter acquire that irregular arrangement referred to by 

 Verworn ('87, pp. Ill and 112). 



Stomach. — Figure 93 (Plate X.) represents a section across the stomach 

 immediately below the cardiac valve, from the same individual as that 

 from which Figure 84 was taken. The proximal ends of all cells stain 

 more deeply than the distal ends, but the cells are all alike as far as re- 

 gards receptivity to stains. Already, in certain regions, the cells are 

 higher or lower than the average, and have even begun to group them- 

 selves as typical ridge- and furrow-cells. Figure 82 is a section through 

 the same region as Figure 93, but from an adult individual. The ridge- 



