1 88 



NORMAL HISTOLOGY. 



FIG. 233. 



its walls also become augmented by many new elements, the meso- 

 dermic cells differentiating into a narrow looser zone next the ento- 



derm, which later becomes the submu- 

 cosa, and a broader, more compact 

 stratum, representing the future mus- 

 cular tunic. The entodermic cells, at 

 first arranged as a single layer, soon 

 undergo local proliferation, the resulting 

 groups of cells disposing themselves as 

 minute cylindrical masses, which are 

 the earliest traces of the peptic glands. 

 These increase in length and later en- 

 croach upon the underlying mesoderm. 

 In the young gland six to eight tubular 

 divisions communicate with a single 

 duct, but as development advances the 

 ducts divide, with a corresponding dimi- 

 nution in the number of terminal com- 

 partments connected with each. The 

 pyloric glands appear about the same 

 time as do the peptic, or at about the 

 tenth week of fcetal life, the cells ac- 

 quiring their characteristic form and 

 appearance during the later stages. At 

 first and during a considerable period 

 the cells lining the peptic glands are 

 all of the same variety ; later certain 

 elements become distinguished by the 

 accumulation of coarse granules within 

 their protoplasm ; these constitute the acid or parietal cells, usually 

 appearing towards the close of the fourth month of fcetal life. 



The intestinal divisions of the primitive gut also depend for 

 their distinctive characters on the differentiation of the entodermic 

 epithelium and of the adjoining mesoderm, which together constitute 

 the mucosa. The villi, distinguishable by the tenth week, are at 

 first relatively short and thick and less numerous than later, when 

 additional projections are developed. It is of interest to note that 

 in the early stages villi appear in both the large and the small intes- 

 tine, these structures subsequently atrophying and disappearing in 

 the large gut while they increase in size and importance in the re- 

 maining parts of the tube. Coincidently with the formation of the 

 villi the entodermic epithelium sends outgrowths into the mesoderm 

 between the villous projections ; these, at first solid, cylinders repre- 

 sent the early stages of the simple tubular glands ; with the gen- 



Sagittal section of nine-day rabbit 

 embryo : B, B 1 , neural canal and brain 

 vesicles; /#, ectodermic invagination 

 which contributes the lining of anterior 

 part of future oral cavity ; p, primitive 

 pharynx, the blind upper end of the 

 primitive gut (g) lined with entoderm, 

 in this stage separated from ectoderm 

 by septum ; U, umbilical duct connect- 

 ing gut with umbilical vesicle ; h, h' , 

 arterial and venous segments of young 

 heart ; delicate endothelial tube seen 

 lying within primitive muscular walls. 



