THE FROG 201 



being small in the summer, but enlarging with the enlargement 

 of the ovaries, and swelling greatly in the early spring pre- 

 paratory to egg laying. In the walls of the oviducts are numer- 

 ous little glands, whose function is to secrete material around 

 the egg to form the shell or other protective covering. They 

 are nidamental glands (Lat. nidus = a nest). 



It will be seen that the sexual organs and the kidneys are 

 very closely connected. They lie close together, have a com- 

 mon opening, and in the male the same duct, the ureter, serves 

 for the exit of the sperms and the urine. A similar close rela- 

 tion is found in other vertebrates, and a study of the develop- 

 ment of the animals shows that their ducts are originally 

 derived from the same organ in the embryo. The two systems 

 together are known as the urogenital system. In the frog 

 this system opens to the exterior with the intestine by the 

 single common cloacal opening. In higher animals they may 

 have separate openings. 



LABORATORY WORK UPON THE FROG 



For a detailed dissection of the frog, reference must be made to some 

 of the numerous laboratory manuals. The brief general directions given 

 below will be sufficient to illustrate the topics discussed in the text, and 

 at least this amount of laboratory work is necessary to make the text 

 properly intelligible. 



If the specimens are obtained alive they should first be killed with 

 chloroform, and, while still fresh, all of the points in the external anatomy 

 should be made out. Note should be made of the following: head; body; 

 .absence of tail; the loose skin, attached, however, at certain points; arms; 

 numbers of fingers; legs; number of toes; web between the toes; mouth; 

 nostrils; eyes with eyelids; ears; cloacal opening. Open the mouth and 

 note tongue; glottis; gullet. 



The dissection of the organs of the abdomen can best be made with a 

 freshly killed specimen, but it may be done satisfactorily with animals 

 preserved in alcohol or formalin. The dissection of the brain and spinal 

 cord should always be made upon animals preserved in alcohol, since 

 these organs are too soft to handle in fresh specimens. A mounted skele- 

 ton of the animal should be at hand for study and comparison with the 

 animal under dissection. 



