302 THE ORGANS 



the inferior vena cava or portal vein. If the vena cava is used, it is convenient to 

 inject from the heart directly through the right auricle into the vena cava. 

 Sections should be rather thick and may be stained with eosin, or even lightly 

 with haematoxylin-eosin (technic i, p. 20), and mounted in balsam. 



(5) For demonstrating the intralobular connective tissue, Oppel recommends 

 fixing fresh tissue in alcohol, placing for twenty-four hours in a 0.5-per-cent. 

 aqueous solution of yellow chromate of potassium, washing in very dilute silver 

 nitrate solution (a few drops of 0.75-per-cent. solution to 50 c.c. of water) and 

 then transferring to 0.75-per-cent. silver nitrate solution, where it remains for 

 twenty-four hours. Embed quickly in celloidin. The best tissue is usually 

 found near the surfaces of the blocks. A similar result is obtained by fixing 

 fresh tissue in 0.5-per-cent. chromic-acid solution for three days, then trans- 

 ferring to o . 5-per-cent. silver nitrate solution for two days. 



Development of the Digestwe System 



In the development of the digestive system all the layers of the blastoderm 

 are involved. Mesoderm and entoderm are, however, the layers most con- 

 cerned, as the ectoderm is used only in the formation of the oral and anal 

 orifices. The primitive alimentary canal is formed by two folds which grow out 

 from the ventral surface of the embryo and unite to form a canal, in a manner 

 that is quite similar to the formation of the neural canal. In this way the 

 primitive gut is lined with cells which previously formed the ventral surface of 

 the embrj'o, i.e., entoderm. A portion of the mesoderm accompanies the 

 entoderm in the formation of the folds. This is known as the visceral layer of 

 the mesoderm. The primary gut is thus a closed sac or tube. It is connected 

 with the umbilical vesicle, but has no connection wath the exterior. These 

 connections are formed later by oral and anal invaginations of ectoderm which 

 extend inward and open up into the ends of the hitherto imperforate gut. 

 The ends of the alimentary tract, including the oral cavity and all of the glands 

 and other structures connected with it, are of ectodermic origin. The epithelial 

 lining of the gut and the parenchyma of aU glands connected with it are derived 

 from entoderm. The muscle, the connective tissue, and the mesothelium of 

 the serosa are developed from mesoderm. 



The mesodermic elements show little variation throughout the gut, the 

 peculiarities of the several anatomical divisions of the latter being dependent 

 mainly on special differentiation of the entoderm (epithelium). Beneath the 

 entodermic cells is a narrow layer of loosely arranged tissue which later separates 

 into stroma, muscularis mucosas, and submucosa. Outside of this a broader 

 mesodermic band of firmer structure represents the future muscularis. 



The stomach first appears as a spindle-shaped dilatation about the end of 

 the first month. Its entodermic cells, which had consisted of a single layer, 

 increase in number and arrange themselves in short cylindrical groups. These 

 are the first traces of tubular glands. They increase in length and extend 

 downward into the mesodermic tissue. For a time the cells lining the peptic 

 glands are all apparently alike, but at about the fourth month the differentia- 

 tion into chief cells and parietal cells takes place. ^ 



