lo GENERAL INTRODUCTION. 



ensuing divisions. From each mother-cell, therefore, arise four spermatozoa, each having half the usual number of 

 single chromosomes. The normal number is restored by union of the two germ-cells in fertilization. 



SUMMARY. 



The foregoing facts may be briefly summarized, as follows : 



I. Fertilization. The union of two germ-cells, a spermatozoon and an ovum, derived from the two respective sexes. 



{a) Entrance of the spermatozoon into the ovum. 

 {b) Union or close association of the two germ nuclei. 

 Meanwhile, 



(i) Transformation of the chromatic substance of each nucleus into a definite number of chromosomes, 

 equal in the two sexes. 



(2) Origin of a centrosome from the middle-piece of the spermatozoon ; formation about it of a sperm-aster. 



(3) Fission of the sperm-aster, and development of a spindle between the two halves to form an amphiaster. 



Grouping of the chromosomes about the equator of the spindle. [Prophases.] The karyokinetic 

 fio-ure formed. 



II. Cleavage. Progressive division or cleavage of the egg. Distribution of the chromatin to the cells of the body, 

 (r) Longitudinal splitting of the chromosomes, and separation of the halves (Metaphase). 



id) Divergence of the daughter-chromosomes to opposite poles of the spindle (Anaphases). 



(c) Reconstruction of two daughter-nuclei from the two groups of daughter-chromosomes, and fission of the entire 



egg (Telophases). 

 {f) Fission of the aster in each daughter-cell, and formation of a karyokinetic figure in each cell, precisely as before. 

 {g) Repetition of cell-division until a multicellular body is formed. Differentiation of the tissues. Origin of the 



germ-cells or their immediate predecessors in the reproductive organs. 



III. Maturation. Reduction of the normal number of chromosomes to one-half. 



( In case of the ovum. Formation of the polar bodies. 



\ In case of the spermatozoon. The last two cell-divisions in the testis (giving rise to four spermatozoa). 



IV. Fertilization. The cycle completed. 



