322 Vertebrate Embryology 



tinue to elongate as solid rods of cells, which 

 later become hollowed out to form the acini 

 and ducts of the gland. Further growth of 

 the gland consists chiefly in the increase in the 

 number and length of the ducts and acini. 

 This growth continues in both sexes until 

 puberty, when it normally ceases in the male ; 

 in the female, at this time, a rapid development 

 of the adjacent dermal tissues, especially the 

 adipose, takes place, forming the breast. The 

 nipple appears at different periods of embryonic 

 life in different individuals ; it sometimes does 

 not appear until after birth. 



In both sexes there is normally a slight secre- 

 tion of milky fluid, witch milk, just after birth. 

 In adult males, cases have been known where 

 the mammary glands were functionally active. 



CALCULATION OF THE AGE OF HUMAN EMBRYOS 



The term " embryo" is usually applied to 

 those stages of development up to about the 

 end of the second month ; after that time it is 

 customary to use the term " foetus." 



The life of the embryo is considered to begin 

 with the fertilization of the ovum, which is 

 supposed to occur in man in the upper third of 

 the Fallopian tube. Ovulation most frequently 



