120 AMPHIBIANS 



cord (Figure 117). In a long salamander there are 20 or 

 30 pairs of nerves on the spinal cord. 



LABORATORY STUDY 



In connection with the study of the frog, the following additional lab- 

 oratory work should be done in order that the several organs of man 

 which are discussed in Part II may be better understood. Frogs that 

 have been preserved in formalin can be easily dissected. Examine the 

 digestive organs : first the mouth, then the esophagus, stomach, small 

 and large intestine, and cloaca. For convenience, the liver will have to 

 be removed. The pancreas can be seen as a small whitish structure in 

 the loop between the stomach and the intestine. The spleen is a round, 

 red organ usually found near the large intestine. 



A pair of narrow kidneys lies close to the back and is connected by 

 ducts with the cloaca. The spermaries are found attached to each kid- 

 ney near the front end and the sperm cells escape to the exterior by the 

 kidney ducts. In the female frog the large ovaries occupy most of the 

 space of the body cavity. A pair of oviducts opens into the body cavity 

 just back of the stomach. The eggs escape from the ovary into the body 

 cavity. 



The nervous system is enclosed in bone that is easily removed from the 

 dorsal surface. The brain should be studied and the following divisions 

 recognized : cerebral hemispheres ending in front in the olfactory lobes, 

 which are not clearly marked. Just back of these the two large roundish 

 optic lobes which are attached to the midbrain (thalamencephalori) , thal- 

 a-men-ceph'a-lon). The cerebellum is small, and the medulla passes into 

 the spinal cord without any sharp dividing line. 



120. Development. — Late in March and early in April 

 the frogs gather in ponds to lay their eggs. The eggs are 

 surrounded by a jelly-like substance which holds them 

 together. As the eggs are being laid by the female frog, 

 the male frog spreads a large number of sperm cells over 

 the whole mass. These sperm cells make' their way 

 through the soft jelly and one of them must enter each 

 egg or it cannot grow into a tadpole. 



As soon as the sperm cell enters the egg (Figure 119), it 

 begins to change from a solid, pointed body into a round 

 nucleus which is so much like the nucleus already in the 



