DIGESTIVE ORGANS. 205 



pupil of the mammals is not always circular, but is a vertical slit in the cats, a 

 horizontal opening in the whales, many ungulates, etc. During development the 

 lids fuse for a time, separating in some cases, only after birth. The edges of the 

 lids are fringed with short hairs, the eye-lashes or cilia, and internal to these are 

 the ducts of sebaceous glands (tarsal or Meiobomian glands), the glands them- 

 selves being in the substance of the lids. The whales have an enormously thick 

 sclera which, here as elsewhere, appears as a continuation of the dural sheath of 

 the optic nerve. 



THE DIGESTIVE ORGANS. 



Few articles of food, as they come to a vertebrate, are in shape to be 

 taken immediately into the organism and to be used, without modifica- 

 tion, as a source of energy or as material for the construction of new 

 tissue or the repair of old. They have to be altered so that they are 

 soluble and so able to pass by osmosis into the blood-vessels (proteids, 

 carbohydrates) , or they must be broken up (hydrocarbons) so as to be 

 taken up by the absorbtive vessels (lacteals) of the lymphatic system. 

 These changes in the food, which are the result of the action of the 

 secretions of the digestive glands, constitute the process of digestion. 

 The digestive tract or alimentary canal, where these changes take 

 place, also has to provide for the passage of the digested food into the 

 blood-vessels, to be carried by them to all parts of the body. It is there- 

 fore richly supplied with blood- and lymph-vessels. 



The alimentary canal, which is complete (i.e., has both mouth and 

 vent), is largely entodermal in origin, but small portions at either end 

 are derived from the ectoderm. The entodermal portion, the mesen- 

 teron, consists of the wall of the archenteron (p. 12) after the separa- 

 tion of the notochord, the mesothelium, and a few less prominent struc- 

 tures. The ectodermal parts are a stomodeum at the cephalic end and 

 a proctodeum behind. 



In the early stages of all vertebrates the mouth is lacking, the ce- 

 phalic end of the archenteron abutting directly against the ectoderm of 

 the ventral side of the head, so that an oral (pharyngeal) plate is 

 formed, consisting of both ectoderm and entoderm. Next this plate is 

 pushed inward, either as a pocket (fig. 190) or as a solid plug, carrying 

 the entoderm before it. This ingrowth constitutes the stomodeum, 

 and the site of its ingrowth forms the mouth opening of the adult. 

 Later the oral plate breaks through, placing the stomodeum and 

 mesenteron in communication. 



