CH. LVill.J DEVELOPMENT. 803 



At each menstrual period the uterus becomes congested, and 

 some of the blood-vessels of the mucous membrane are ruptured ; 

 the blood, together with the secretion of the glands and some 

 epithelial debris from the surface, constitutes the menstrual flow, 

 which usually lasts two or three days. The amount of destruction 

 of the surface epithelium is not, however, a marked phenomenon ; 

 still less is there any disintegration of the deeper parts of the 

 mucous membrane. 



For a description of the mammary glands see p. 460. 



CHAPTER LVIII. 



DEVELOPMENT. 



THE description of the origin and formation of the tissues and 

 organs constitutes the portion of biological science known as 

 embryology. This subject is a large one, and many books are 

 written which have for their exclusive object its elucidation. All 

 one can possibly attempt in a physiological text-book is the 

 merest outline of the principal facts of development. 



In our descriptions throughout we shall endeavour to speak of 

 the development of the mammal principally ; it will not be 

 possible to do so altogether, for many of the facts which are 

 believed to be true of the mammal (man included) have only been 

 actually seen in the lower animals. That they occur in the 

 higher animals is a matter of inference. 



It will, however, add interest to the subject to draw some of 

 our descriptions from the lower animals ; for the scientific dis- 

 cussion of embryology must always start from a wide survey of 

 the whole animal kingdom. Without entering into any argu- 

 ments in relation to the Darwinian theory of evolution, it will be 

 sufficient to state in general terms that the series of changes which 

 occur in the embryological history of the highest animals, form a 

 compressed picture of the changes which have taken place in their 

 historical development from lower types. 



The Ovum. 



The human ovum is like that of other mammals, a small 

 spherical body about T ^-y to -^\ v inch in diameter ; it is an 

 organised animal cell consisting of a mass of protoplasm enclosing 

 a nucleus and attraction sphere. The protoplasm, however, also 



3 K 2 



