198 



GENERAL BIOLOGY 



glands, they are differentiated into bulky cells that secrete 

 the digestive fluids at the bottom, mucus-secreting cells 

 along the sides of the pits and of protective cells about the 

 mouths of the pits where they come in contact with the 

 food. In the lung, where the extension of the walls is very 

 great, the cells are spread out flat and very thin to cover 

 them. This thinness favors the diffusion of gases between 

 the blood and the air, and is characteristic of respiratory 

 epithelium. 



The liver arises very 

 early (fig. 123) as a wide 

 sacculation of the archen- 

 teron near its anterior end 

 on the ventral side. This 

 outgrowth becomes r e- 

 peatedly branched, and 

 the resulting glandular 

 blind tubules become con- 

 voluted, the basal connec- 

 tion remains as a slender 



*, enteron; the black spur from the upper rO nnprtinp- tubp the bile 



side represents the free part of the noto- COnnCC ting TUDC, tne 



cord. The ventral outgrowth from the A-,-,^ c>nr\ -t-Vio rlicf a1 rr-H-irvn 



front end will form the liver. OUCt, and the distal portion, 



acquiring through meso- 



dermal additions a system of blood vessels, becomes differ- 

 entiated into the lobes of the liver. A dilatation on the bile 

 duct becomes the gall cyst (fig. 114). The pancreas arises 

 later but by a simpler and somewhat parallel development 

 arrives at its adult estate, when it consists of a mass of 

 glandular-walled communicating tubes, which secrete the 

 most important single digestive fluid of the body. The uri- 

 nary bladder arises as a similar but simpler sacculation at 

 the posterior end. 



Development of the neural tube. We have already seen 

 that -the central nervous axis develops into a tube by the 



FIG. 123. Diagram of a longitudinal sec 

 tion of a young embryo, n, neural tube 



