566 ZOOLOGY. 



opens in the female immediately below the anus, but in the 

 male enters the penis. 



The ovary (Ov) is small and is placed near the open end 

 of the oviduct or Fallopian tube, which can be seen in the 

 figure extending alongside the rectum above the bladder. 

 The two oviducts (Ovd) unite posteriorly to form the uterus 

 (lit). 



Fig. 492, II., is a median longitudinal section of the brain. 

 The spinal cord passes into the medulla oblongata (M), over 

 which lies the large cerebellum (Cb), and the small corpora 

 quadrigemina (Q). In front is the large cerebrum (C) and 

 the small olfactory lobe (L). Fig. 492, III., is a diagram of 

 the eye (see explanation of the figure). 



By carrying the dissection further, the student will be able 

 to examine the tongue with its papillae ; the epiglottis at 

 the back of the mouth in front of the trachea ; the larynx, 

 a peculiarly modified portion of the trachea in the neck, 

 with two elastic bands stretched across its interior ; the 

 bands or vocal cords may be set in vibration by a blast of 

 air from the lungs. The heart may also be dissected fur- 

 ther to find the origin of the pulmonary vessels, and to> 

 make out the four divisions or chambers. (Minot.) 



The eggs of mammals are exceedingly minute, partly owing 

 to the small quantity of yolk in them ; the eggs of the few 

 which have been examined are about a quarter of a milli- 

 metre ( T J-g j^-g- inch) in diameter. In the duckbill the egg 

 is large and with more yolk, like those of birds, being about 

 five millimetres in diameter. Mammals are divided into 

 non-placentals and placentals, according as the embryos are 

 surrounded or not with a placen ta or "after-birth." This 

 organ is a development of the allaiitois, serving as a means 

 chiefly of nutrition, being filled with blood-vessels leading 

 from the walls of the womb of the parent, and also acting 

 as an organ of respiration, and to carry off the effete pro- 

 ducts by means of the maternal circulation. 



Mammals may be born helpless and only partly developed, 

 as in the Marsupials ; or capaole of locomotion and sucking 

 milk, as in the calf or colt ; or helpless for many months, 

 as in human infants. The changes in the form of the body 

 after birth are much less, on the whole, than in the birds. 



The sexes differ externally in size and ornamentation. 



