BE PR OD UCTION. 45 1 



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 their 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 

 given rise to a large amount of discussion. Maturation begins approximately 

 as the ovum is leaving the ovary, and is not completed until after the ovum 

 has received the spermatozoon, although the exact time-relations in the 

 human species are not yet determined. It consists of a mitotic division of 

 the nucleus, essentially like mitosis (karyokinesis) in ordinary cell-division, and 

 an expulsion of one portion from the cell. This occurs twice in succession. 

 The cast-off bits of protoplasm are known as polar bodies. The details of the 

 process of maturation are as follows (Fig. 221) : In all animals the nucleus 

 of the ovarian ovum, or oocyte, at the time of its formation receives from its 

 mother-cell the same number of chromosomes as the ordinary tissue-cells con- 

 tain. These constitute its chromatic reticulum. As the oocyte prepares for 

 maturation the chromatin is resolved into masses, the number of which is one- 

 half that of the somatic chromosomes. In a large number of species, such as 

 many of the worms, the insects, and the crustaceans, each chromatic mass 

 constitutes a group of chromosomes, usually four in each group, which is 

 called a "quadruple-group" or "tetrad" (/>). The number of tetrads is 

 hence one-half the number of original chromosomes, while the total number 

 of chromosomes in the nucleus at this stage is double the original number. 

 The nucleus moves from its position in the interior of the egg toward the sur- 

 face, and the nuclear membrane begins to disappear. At the same time the 

 two minute cytoplasmic structures, the centrosomes, which lie close beside 

 the nucleus, separate and take up positions at a considerable distance apart 

 from each other, in some cases even upon opposite sides of the nucleus. The 



