CHAPTER XVII 



DEVELOPMENT 



Changes Which Occur in the Ovum Prior to Impregnation. The 



ovum when ripe and detached from the ovary is a single cell enclosed within 

 the zona pellucida, and containing the germinal vesicle and germinal spot. 

 The ovum undergoes a series of changes preparatory to fertilization, known 

 as maturation, the general effect of which is to reduce the chromatin in anticipa- 



FIG. 499. The Maturation of the Ovum; Extrusion of the "Polar Bodies." (Diagrammatic.) 

 A, An ovum at the commencement of the process; B, after the formation of the spindle. The 

 chromosomes are gathered at the equator of the spindle. C, One apex of the spindle has pro- 

 jecteJ into a bud on the surface, and half of the divided dyads have passed to each pole; D, the 

 separation of the first polar body; E, the commencement of the second polar body; F, the comple- 

 tion of the second polar body. (Cunningham.) 



tion of the added chromatin from the sperm nucleus. The primary change 

 observed in the ovum consists in the migration of the germinal vesicle or nucleus 

 to the surface, and the disappearance of its nuclear membrane, with a con- 

 sequent indistinctness of its outline. Its protoplasm becomes to a consider- 

 able extent confounded with the yolk substance, and its germinal spot dis- 

 appears. The next step in the process is the appearance in the yolk of two 



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