35O THE GENITO-URINARY ORGANS. 



granules ; in the former lies the germinal vessel. Between the 

 protoplasm of the ovum and the zona pellucida is found a narrow 

 space known as the perivitelline space. The germinal vesicle 

 (nucleus), which is usually of spheric shape, possesses a doubly 

 contoured membrane and a large germinal spot (nucleolus), which 

 shows ameboid movements. 



The origin of the zona pellucida has not as yet been fully de- 

 termined. It probably represents a product of the egg epithelium, 

 and may be regarded in general as a cuticular formation of these 

 cells. At all events it contains numerous small canals or pores into 

 which the processes of the cells composing the corona radiata ex- 

 tend. These processes are to be regarded as intercellular bridges 

 (Retzius, 90) ; and, according to Palladino, they occur not only 

 between the ovum and the corona radiata, but also between the 

 follicular cells themselves. In the ripe human ovum the pores are 

 apparently absent (Nagel), and it is very probable that they have to 

 do with the passage of nourishment to the growing egg. Retzius 

 believes that the zona pellucida is derived from the processes of the 

 cells composing the corona radiata, which at first interlace and form 

 a network around the ovum. Later, the matrix of the membrane is 

 deposited in the meshes of the network, very probably by the egg 

 itself. 



Further developmental changes are, however, necessary before a 

 fully developed ovum (ripe ovum) may be fertilized. These are 

 grouped under the head of maturation of the ovum. They have in 

 part been described in a former section (p. 71), but may receive 

 further consideration at this time. During maturation the chromo- 

 somes are reduced in number, so that the matured ovum presents 

 only half the number found in a somatic cell of the same animal. 

 The manner in which this reduction takes place has been described 

 for many invertebrates and vertebrates, and in all ova studied with 

 reference to this point essentially the same phenomena have been 

 observed. In this account we shall follow the process as it occurs 

 in the Copepoda (Riickert, 94). 



During the period of growth the cells composing the last gen- 

 eration of oogonia (primitive ova) increase in size, and are then 

 known as " oocytes " (the ripe ova). These then undergo mitotic 

 division, and in each a spirem is formed which divides into 12 

 chromosomes, and not into 24 as in the case of the somatic cells. 

 These 12 chromosomes split longitudinally, so that the germinal 

 vesicle is seen to contain 12 pairs of chromosomes, or daughter 

 loops. By this process the oogonia have become egg mother cells 

 (O. Hertwig, 90) or oocytes of the first order. The loops now 

 begin to shorten and each soon divides crosswise into two equal 

 rods, thus giving rise to 12 groups of 4 chromosomes, or 12 tetrads. 

 The mother cell now divides into 2 unequal parts, the process con- 

 sisting in a distribution of the rods composing the tetrads in such a 



