REPRODUCTION. 889 



The chromatin is peculiar to the nucleus, and at certain stages in the nuclear 

 history is resolved into distinct granules or filaments^ the chromosomes (Fig. 

 308, A), the number of which in the human ovum is unknown. There 

 is much reason for believing that the chromatin is the bearer of whatever is 

 inherited from the mother. The nucleus is limited by a nuclear membrane, 

 and contains a strongly marked nucleolus, which has likewise retained its 

 original name of germinal spot. There is probably no proper cell-wall, or 

 vitelline membrane, such as is said to exist in many mammalian and other eggs. 

 The ovum is, however, surrounded by a thick, tough, transparent membrane of 

 ovarian origin, about 0.02 millimeter (y^Vrr inch) in thickness, and called the 

 zona radiata or zona pellucida (Fig. 307, z). It is pierced by a multitude of fine 

 lines radiating from the surface of the zona to the ovum ; these are thought 

 to represent pores, to contain fine protoplasmic processes of the surrounding 

 ovarian cells, and thus to serve as channels for the passage of nutriment to 

 the egg. Between the zona radiata and the ovum a narrow space, the peri- 

 vitelline space (s), exists. Attached to the outside of the zona radiata are 

 usually patches of cells derived from the discus proligwus of the Graafian fol- 

 licle of the ovary, which may form a complete covering and constitute the corona 

 radiata. They disappear soon after the egg is discharged from the ovary. 



Regarding the chemistry of the mammalian ovum little is known definitely, 

 and of the human ovum nothing whatever except by inference from the eggs 

 of lower animals. The protoplasmic basis undoubtedly resembles other undif- 

 ferentiated protoplasm in its general composition, with an abundance of proteid 

 among its solid constituents. Deutoplasm is a rich mixture of food-substance 

 in concentrated form, and contains among its solids probably vitellin, nuclein, 

 albumin, lecithin, fats, carbohydrates, and inorganic salts. 



The form and the structure of the egg suggest the part that it plays in 

 reproduction. It is not locomotor ; in fertilization it is the passive element ; 

 it remains in its place and is sought by the spermatozoon. Its nucleus is the 

 equivalent of that of the spermatozoon. Its form renders easy the entrance of 

 the male element. Its bulk consists largely of food in a very concentrated 

 form, and, as development proceeds, it supplies this food to the growing cells. 



In lower forms of animal life, where eggs are fertilized outside the body 

 of the parent in the water into which they are set free, they are usually pro- 

 duced in enormous numbers. Some fail of fertilization, while others are 

 destroyed by enemies, and the large number is a compensatory adaptation by 

 nature for then poor chance of survival. In mammals and man, however, 

 ova have a much better opportunity of being fertilized and of developing into 

 adults, and their number is correspondingly reduced. Their relative fewness, 

 as compared with the spermatozoa, is in harmony with their larger size and 

 the fact that, while awaiting fertilization, they are carefully protected within 

 the body of the mother. 



Maturation of the Ovum. Attention has been called to the maturation 

 of the spermatozoon. The ovum undergoes an analogous process of ripening, 

 which has been studied very carefully, and from its theoretical interest has 



