DEVELOPMENT OF THE OVUM. 



87 



become of the polar bodies in the course of development of the 

 fertilized ovum is not known. 



Zona radiata 

 polar bodies 



Zona radiata 



Zona radiata 



A 



B 



Fig.' 65. — Showing the production of the Morula from the Ovum. 

 A. The ovum after the first division. B. After the second. C. The Morula 



In the course of fecundation thousands of spermatozoa are 

 lodged in the genital passage ; many stem the adverse current of 

 the uterine cilia, reach and live for days within the interlaminar 

 grooves in the wider parts of the tube. In the course of its 

 descent within one of the grooves the egg may be fertilized. The 

 spermatozoon bursts through the zona radiata, loses its tail, its 

 head enlarges, and forms the male pronucleus. The male and 

 female pronuclei unite, and from their union springs the nucleus 

 of the fertilized ovum. This is the centre from which all future 

 developmental changes start. The ovum may be, but rarely is, 

 fertilized in the ovary, or between the ovary and ostium ab- 

 dominale, the result being a pelvic gestation. The length of 

 time the fertilized ovum takes to reach the uterus is not known 

 exactly, but probably it spends about ten or twelve days within 

 the Fallopian tube, and during that time it passes through the 

 following stages : 



1. The Morula. — The ovum, with a full display of karyoki- 

 netic changes in the nucleus, divides, subdivides, and grows 

 within the zona radiata. until a rounded mass of cells is formed 

 — the morula or mulberry mass. The cells' are of unequal size 

 and divide at unequal rates (Fig. 65, A, B, C). 



2. The Blastodermic Vesicle. — This stage is produced from 



