680 COLLEGE ZOOLOGY 



for capturing fish (compare teeth of perch, p. 437); the carniv- 

 orous animals, like the dog (Fig. 518), are provided with large 

 canine teeth for capturing and killing their prey, small and almost 

 useless incisors, and molars with sharp edges for cutting or crush- 

 ing; herbivorous animals, like the ox, possess broad incisors for 

 biting off vegetation, no canines, and large grinding molars 

 (Fig. 546, V); rodents, like the rabbit (Fig. 511), have incisors 

 that grow throughout life, but are worn down by gnawing, thereby 

 maintaining a serviceable length and a keen cutting edge; in- 

 sectivores, such as the shrew (Fig. 516), seize insects with their 

 projecting incisors and cut them into pieces with the pointed 

 cusps on their premolars and molars ; and man and other omniv- 

 orous animals are provided with teeth fitted for masticating 

 both animal and vegetable matter. 



c. The Development of Mammals 



The eggs of most mammals develop within the body of the 

 mother; the exceptions are the monotremes (p. 645), which lay 

 eggs. During their development the eggs of mammals, as well 

 as those of birds and reptiles, produce two membranes: (i) the 

 amnion, and (2) the allantois. Because of the presence of these 

 membranes, the mammals, birds, and reptiles are often grouped 

 together as AMNIOTA, while the amphibians, fishes, elasmo- 

 branchs, and cyclostomes, w r hich do not possess these mem- 

 branes, are designated as ANAMNIOTA. 



The segmentation of mammals' eggs is complete (except in 

 monotremes), and takes place either in the oviduct, as in the 

 rabbit, or in the uterus, as in the sheep. Figure 547 illustrates 

 by a series of diagrams the formation of the embryonic mem- 

 branes of a mammal. The processes are briefly noted beneath 

 the diagrams. 



The placenta which is present in some marsupials and all the 

 other EUTHERIA arises in the following manner. " In the uterus 

 the embryo becomes connected with the uterine wall by means 

 of its outer epithelial layer, now known as the trophoblast. 



