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 antici- 



FIG. 499. The Maturation of the Ovum; Extrusion of the "Polar Bodies." (Dia- 

 grammatic.) 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 projected 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 completion of the second polar body. (Cunningham.) 



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

 observed in the ovum consists in the migration of the germinal vesicle or nu- 

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

 consequent indistinctness of its outline. Its protoplasm becomes to a con- 

 siderable extent confounded with the yolk substance, and its germinal spot 



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