218 NORMAL HISTOLOGY. 



general or " somatic " cells of the body (Fig. 193). The polar bodies 

 perish, as does also the ovum, unless fertilized by the introduction 

 of a spermatozoon. The latter, as we shall see, also contains half 

 the number of chromosomes contained in the somatic cells ; so that 



FIG. 193. 





Maturing ovum of physa (fresh-water snail). (Kostanecki and Wierzejski.) Above are the 

 two small cells resulting from the division of the first polar body. Below is the ovum, 

 the nucleus of which is dividing to form the second polar body. Near the centre of the 

 ovum is the nucleus of the spermatozoon, just above which is its (divided) centrosome 

 with surrounding radiations in the cytoplasm. When the second polar body has been 

 formed the chromosomes remaining in the ovum will be ready to participate with those 

 of the spermatozoon in the further development of the then fertilized egg. 



after its entrance into the mature ovum the latter acquires its full 

 complement of chromosomes and is ready for development. 



The Mammary Gland. Each mamma consists of a group of about 

 twenty similar compound racemose glands, opening by distinct orifices 

 at the tip of the nipple, and separated and enclosed by fibrous tissue, 

 in which there is a variable amount of fat. At the edges of the 

 mamma this fibrous stroma becomes continuous with the tissues of 

 the superficial fascia in which the breast is situated. 



Each of the glands entering into the composition of the breast 

 possesses a single main duct, the " galactiferous duct," which is lined 

 with columnar epithelium, except near its orifice, where the strati- 



