250 ANATOMY FOR NURSES. [CHAP. XX, 



called the vitellus or yolk ; (3) imbedded in the vitellus or yolk 

 is a transparent, sharply outlined nucleus, the germinal vesicle ; 

 and (4) in the germinal vesicle is a small dark nucleolus, the 

 germinative spot. 



It is impossible for us to trace the growth and development 

 of a fecundated ovum. The subject is too complicated for us 

 to attempt to describe it in a book of this kind, and we shall, 

 therefore, content ourselves with briefly describing the first two 

 or three steps. 



Soon after leaving the ovary, the germinal vesicle and ger- 

 minal spot in a fecundated ovum disappear, and the protoplasm 

 begins to divide inside the vitelline membrane into two halves, 

 in each of which appears a nucleus. The halves divide into 

 quarters, the quarters into eighths, and so the subdivision con- 

 tinues until a great number of minute cells are produced, which 

 soon arrange themselves, close to each other like bricks in a 

 wall, upon the inner surface of the vitelline membrane. The 

 cells thus in close contact with one another form a membrane, 

 called the epiblast. Upon this membrane a second one soon 

 appears, formed in the same way and lining its inner surface. 

 This is called the hypoblast. Subsequently a third membrane, 

 the mesoblast, is developed between the epi- and hypoblast, and 

 from these three membranes all the tissues and complicated 

 structures of the body are evolved. 



Upon the arrival of the ovum in the uterus, it is grafted 

 upon the mucous membrane. It usually lodges upon the upper 

 surface of the uterus, between two folds of the mucous lining, 

 which soon grow up all around it, and, as it were, bury the germ 

 in a circular grave. From the thickened mucous membrane 

 lying between the ovum and the uterine wall, the placenta is 

 ultimately formed for the nourishment of the embryo. 



The mammary glands. The mammary gland is a compound 

 gland, formed of branching ducts ending in secretory recesses. 

 The whole organ is divided by connective tissue partitions into 

 a number of lobes, each of which possesses its own excretory 

 duct opening by a separate orifice upon the surface of the 

 nipple, the gland being in fact not a single gland, but several 

 glands bound together. Just before opening on to the nipple, 

 each excretory duct is widened into a flask-shaped enlarge- 

 ment. 



