4 THE ANATOMY OF VEETEBEATED ANIMALS. 



part of wtat is called the placenta, and may be developed from, 

 the umbilical vesicle (as in Mustehis among Sharks), or from 

 the allantois and chorion (as in most mammals). At birth, 

 they may be either simjjly detached from the substance of 

 the parental organism, or a part of the latter may be thrown 

 off along with them and replaced by a new gro-wth. In 



FiK- 1- 





i'i'^- 1.— Diagrammatic section of the pregnant uterus of a dcciduate 

 placental mammal (Humo): u, uterus; /, Fallopian tube; c, neck 

 of the uterjis ; dn, uterine decidua : ./.«, d/'cidiia xerotina ; dr, dfcidnn 

 rrfJexa ; z. z\ villi ; ch, chorion; am, amnion ; nh, umbilical vesicle ; 

 al. allantois. 



the highest vertebrates, the dependence of the young upon 

 the parent for nutrition does not cease even at birth ; but 

 certain cutaneous glands secrete a fluid called milk, upon 

 which the young is fed for a longer or shorter time. 



When development takes place outside the body, it may 

 be independent of parental aid, as in ordinary fishes ; but. 

 among some reptiles and in most birds, the parent supplies 



