492 COLLEGE ZOOLOGY 



oviduct (7) through a small opening (2), and pass down into the 

 thin-walled, distensible uterus. The glandular wall of the ovi- 

 duct secretes the gelatinous coats of the eggs. The fertilization 

 and development of the eggs will be described later (pp. 506- 



5io). 



Just in front of each reproductive organ is a yellowish, glove- 

 shaped fat-body (Fig. 416, A, 7; B, 4) which serves to store up 

 nutriment. 



Glands. — Besides the liver and pancreas, there are a number 

 of glands in the body of the frog that are of great importance 

 because of their secretions. These glands have no ducts, but 

 empty their products directly into the body; they are therefore 

 called ductless glands, and their products are called internal 

 secretions. Internal secretions are also produced by other 

 organs, e.g. the liver forms sugar and urea. 



The spleen is a reddish body situated above the anterior end 

 of the cloaca. In it old blood corpuscles are destroyed and new 

 colorless corpuscles are probably formed. 



The two thyroid glands are situated one on either side of the 

 hyoid. Their secretions contain a large amount of iodin. The 

 function of the thyroid is not certain in the frog. In man its 

 atrophy causes a disease called cretinism. 



The two thymus glands lie one behind each tympanum, be- 

 neath the depressor mandibular muscle. Their function is not 

 certain. 



The adrenal bodies are long, thin glands lying on the ventral 

 surface of the kidneys. They secrete adrenalin, a substance 

 necessary for the life of the animal. When adrenalin is ex- 

 tracted and then injected into the blood of a mammal, it causes 

 a contraction of the blood-vessels and therefore raises the blood 

 pressure. 



The Skeleton. — The skeleton of the frog consists principally 

 of bone. The axial portion comprises the "skull and vertebral 

 column. The appendicular portion consists of the pectoral and 

 pelvic girdles and the bones of the limbs which they support. 



