REPRODUCTIVE ORGANS IN ANIMALS 35 



provided with unstriated muscle fibres, the contraction of wliich 

 assists in the passage of the milk. The mammary glands are 

 well supplied with blood vessels and lymphatics. 



The nipples are formed of areolar tissue with unstriated 

 muscle fibres. In the cow and many other animals the muscles 

 are especially developed at the end of the nipple, where they 

 form a sphincter which, when contracted, " holds up " the milk. 

 The nipples also contain blood vessels which render them erectile. 

 On the surface are sensitive papillae. Around each nipple in 

 man there is a small area of pink or pigmented skin containing 

 minute glandular organs. 



The purpose of the nipples is to facilitate the young in draw- 

 ing off the milk. This is done by sucking, the nipple being 

 enclosed by the lips of the young while the tongue of the latter 

 is decreased in size in front and increased behind. The 

 composition of the milk is such that it is a complete and perfect 

 food for the young during the early stages of life outside of the 

 mother's bodv. 



The Maturation of the Germ Cells ^ 



It may be convenient here to refer briefly to the maturation 

 processes which are essentially similar in both spermatogenesis 

 and oogenesis. In each case they result in the reduction of the 

 nuclear material or chromatin to one-half the normal amount 

 characteristic of the body cells of the species in question. The 

 chromatin is continued in certain filaments termed chromosomes, 

 and is so called because it undergoes a more intense reaction under 

 the influence of dyes of various kinds when these are applied to it. 

 The number of chromosomes is usually constant for the cells of 

 any particular plant or animal, though it may vary in the two 

 sexes, the cells of one sex containing one more chromosome than 

 those of the other sex (or the chromosomes in the two sexes may 

 be equal in number, but certain of them different in kind). Thus 

 in man there are said to be forty-eight chromosomes in all the 

 body cells (or, according to Painter, forty-seven in the male and 

 forty-eight in the female), excepting in the mature germ cells 

 (ova and spermatozoa), and in the last stage leading up to these, 

 where there are twenty-four. (According to Painter, however, 



