974 THE PHYSIOLOGY OF REPRODUCTION. 



being guided in their movements very probably by the opposing 

 force of the ciliary contractions in the uterus. It is known that 

 the cilia of the tubes and Uterus contract so as to drive inert 

 objects toward the vagina and they carry the egg in this direction, 

 but the spermatozoa, being moved by the contractions of their 

 own cilia or tails, are stimulated to advance against this ciliary 

 current. The act of fertilization of the ovum is preceded by 

 certain preparatory changes in the ovum itself which are described 

 under the term maturation. 



Maturation of the Ovum. The process of maturation occurs 

 before or just after the spermatozoon enters the ovum. At the 

 time the latter is extruded from the follicle it is a single cell sur- 



rounded by a layer of fol- 

 ilcular epithelium forming 

 the corona radiata, which 

 is subsequently lost. The 



/ egg proper consists of cyto- 



/ d plasm and a nucleus or 



germinal vesicle containing 

 a nucleolus or germinal 

 J~ n spot. Within the cyto- 

 plasm is a definite collec- 

 tion of food material or 

 yolk which is sometimes 

 designated as deutoplasm. 

 The whole structure is sur- 



Fig.302. Human ovum (Lee, modified from rounded by a membrane 



kn wn as the zona radiata 



' 302 )' 



the egg reaches the Fal- 

 lopian tube its nucleus undergoes the changes preparatory to 

 a mitotic division. The changes that occur in an ordinary cell 

 division are represented schematically in Fig. 303. The nucleus 

 at first presents the ordinary chromatin network, and in the 

 cytoplasm lies the minute structure known as the centrosome. 

 This latter divides into two daughter-centrosomes (b) which move 

 to opposite sides of the nucleus and become surrounded by rays, 

 each centrosome with its radiating system forming an astro- 

 sphere. The chromatin material in the nucleus meanwhile has 

 collected into larger threads known as chromosomes (c), and 

 the nuclear membrane disappears (d). The number of chromo- 

 somes is definite for each species of animal: The chromosomes 

 arrange themselves equatorially between the astrospheres and then 

 each divides longitudinally into two parts (/) . These parts migrate 

 or are drawn toward their respective centrosomes (gr, h, i), and this 



