SEGMENTATION 385 



on which the chromosomes are again arranged as before, and 

 the second polar cell is formed. These polar cells disintegrate. 

 The nucleus of the ovum now contains only about half the 

 number of chromosom(>s present before the process of throwing 

 off the polar bodies. This remaining portion of the nucleus 

 retires from the circumference and is called the female pro- 

 nucleus, or macrogamete, which is ready for fertilization. The 

 spermatozoon previous to entering the ovum undergoes a 

 similar reduction of chromosomes, though the resulting cells 

 form sperms. 



Fertilization. — The enveloping membrane of the ovum con- 

 tains one or more minute openings through which the sper- 

 matozoon, or male germ cell, enters for the purpose of fertiliza- 

 tion. The spermatozoon consists of a nucleus or extremely 

 minute head, a centrosome, and a flagellum or tail, which is 

 for the purpose of locomotion and disappears upon the entrance 

 of the spermatozoon into the ovum. The nucleus, called the 

 male pronucleus or microgamete, enlarges, the centrosome 

 divides, and a mitotic figure is formed which moves toward the 

 female pronucleus, which moves toward the male pronucleus. 

 Finally, male and female nuclei meet in the midst of the spindle 

 formed about the male nucleus; thus the male and female 

 nuclei are united into one nucleus or zygote. 



Segmentation. — This zygote or fertilized ovum now contains 

 the normal number of chromosomes or "hereditary threads" 

 for its species, one-half of which have been furnished by the 

 maternal cell and one-half by the paternal cell. When the 

 process of mitosis is completed, this zygote is divided into two 

 daughter-cells, each containing the same number of chromo- 

 somes, half of which have been derived from the sperm and half 

 from the ovum. Each of these daughter-cells subdivides into 

 two, and the resulting four, into eight cells, and so on, the num- 

 ber varying with different species. When there is little or no 

 yolk or nutritive material the whole egg divides equally; when 

 there is much yolk the division is unequal or partial. 



Differentiation of Tissues. — From the ectoderm are produced 

 the outer portion of the skin and its outgrowths and the whole 

 nervous system; from the endoderm come the lining of the di- 

 gestive tract and the essential parts of the glands connected with 



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