286 INEZ AYHIPPLE WILDER. 



Although there is at the time of hatching a considerable cavity 

 within the yolk mass (Figs. 10, b, and 12, a), its walls are irregular 

 and poorly defined. The anterior region of the cavity is located 

 more upon the right side and communicates with the lumen of 

 the duodenum, while more posteriorly the cavity swings some- 

 what to the left side, where it finally communicates with a partly 

 differentiated posterior region of the intestine which extends 

 transversely from left to right to lead into the rectum. Thus the 

 5-form so soon to appear in the external moulding of the in- 

 testine (Fig. 12, r) is already indicated in the lumen of the yolk 

 mass. 



The further differentiation of the intestine from the mass of 

 yolk cells (Fig. 13) involves a double process consisting of (i) 

 rapid reduction and disappearance of the yolk material contained 

 in the nutritive cells (yl), which may from time to time be 

 sloughed off into the lumen, or may be ruptured, thus discharging 

 their contents into the lumen; and (2) the proliferation of the 

 peripheral cells (pr), the planes of division being radial with 

 relation to the lumen of the intestine, and resulting in the 

 formation of the anlagen of the future folds of the mucous 

 membrane, in the form of groups of tall columnar cells which, 

 elongating, push their way to the lumen between the remaining 

 nutritive cells (Fig. 13, c}. These nutritive cells thus come to 

 lie in the furrows between the folds, from which place they grad- 

 ually disappear. Mucous cells develop early from certain of 

 the peripheral cells, and are of a tall columnar type, corresponding 

 to the form of the other epithelial cells among which they lie. 

 Multicellular glands (gl) develop later from a proliferation of 

 cells at the bases of the folds. 



Although this differentiation of the intestine progresses almost 

 simultaneously throughout the main part of the yolk mass, it 

 proceeds a little more rapidly at both anterior and posterior ends, 

 than in the middle of the yolk region. Thus at the time of 

 hatching the anterior part of the duodenum and practically all 

 of the rectum are fully differentiated, and the regions of intestine 

 immediately adjoining these are in that stage of development in 

 which the anlagen of the intestinal folds are already formed, a 

 condition which is not reached by the larger portion of the 

 intestine 'until several days later. 



