MAMMALIAN EMBRYOLOGY 



633 



If the student has thoroughly mastered the subject matter of this 

 semester's work in embryology he will not only 

 be able to understand how a normal embryo de- 

 velops, but he will also know how and why many 

 and varying types of abnormal development oc- 

 cur by either mechanical or chemical injury of 

 some kind, which injury may cause any portion 

 of the embryo to stop growing, while other parts 

 continue in the usual manner. Monstrosities of 

 many kinds may thus be formed, and even in ap- 

 parently normal individuals it is by no means 

 rare for the surgeon to find individual internal 

 organs underdeveloped or overdeveloped. All 

 such deviations from the normal are of the ut- 

 most importance to the medical man, and it is 

 only through a study of embryology that they are 

 made understandable. 



Fig. 365. 



Figure to illustrate the 

 ''vertex-breech" method of 

 measuring human embryos. 

 a-b, vertex-breech length of 

 the embryo. 



