ORIGIN OF THE INTERNAL ORGANS. 223 



Becoming constricted in the middle, the fore-gut next 

 appears to consist of two dilatations of about equal size, 

 connected by a narrower passage (fig. 57, E, fg^, fcf). 

 The front dilatation becomes the oesophagus and tlie 

 cardiac division of the stomach; the hinder one, the 

 pyloric division. At the sides of the front end of the 

 cardiac division two small pouches are formed shortly 

 after birth ; in each of these a thick laminated deposit 

 of chitin takes place, and constitutes a minute crab's- eye 

 or gastrolith, which has the same structure as in the 

 adult, and is largely calcified. This fact is the more 

 remarkable as, at this time, the exoskeleton contains very 

 little calcareous deposit. In the position of the gastric 

 teeth, folds of the cellular wall of corresponding shape 

 arc formed, and the chitinous cuticle of which the teeth 

 are composed is, as it were, modelled upon them. 



The hind-gut occupies the whole length of the abdo- 

 men, and its cells early arrange themselves into six 

 ridges, and secrete a cuticular layer. 



The mid- gut, or hypoblastic sac, very soon gives off 

 numerous small prolongations on each side of its hinder 

 extremity, and these are converted into the caeca of the 

 liver (fig. 57, E, mg). The cells of its tergal wall are in 

 close contact with the adjacent masses of food-yelk ; and 

 it is probable that the gradual absorption of the food- 

 yelk is chiefly effected by these cells. At birth, however, 

 the lateral lobes of the food -yelk are still large, and 

 occupy the space left between the stomach and liver 



