DEVELOPMENT. 183 



owing to the fact that both the female and male elements may 

 remain in the genital passages of the female for some days in a 

 healthy condition, it is difficult to fix the time of actual impreg- 

 nation of the ovum. 



What facts lead to the supposition that impregnation takes place 



most commonly before menstruation ? 



(1) It is probable that in most instances the rupture of the 

 Graafian follicle occurs very early in the menstrual period. (2) 

 The uterus is in the most favorable condition to sustain the fecund 

 ovum at that time, because of the presence of the decidua men- 

 strualis. (3) Among the Jews, a remarkably prolific race, coitus 

 is prohibited by the religious law for a week after menstruation. 



DEVELOPMENT. 

 What changes occur in the ovum before impregnation? 



The ovum, after leaving the ovary and while still in the Fallo- 

 pian tube, undergoes certain changes. These changes are mostly 

 confined to the nucleus or germinal vesicle. The germinal spot or 

 nudeolus disappears, and the outline of the vesicle becomes indis- 

 tinct, elongated so that it has a spindle-shape. While in this con- 

 dition the spindle develops two or three vesicles, which collectively 

 form the female pronucleus. 



What changes in the ovum follow impregnation? 



The head of a spermatozoon penetrates the vitelline membrane 

 and enters the vitellus, where it remains surrounded by protoplasm. 

 This is the male pronucleus. The male and female pronuclei soon 

 join, and then follows a process of growth by the splitting up of 

 the nuclei to form new nucleated protoplasmic cells. 



What is this splitting-up process called? 



Segmentation. The first cell which is formed by the union of 

 the male and female pronuclei is called the blastosphere or primi- 

 tive segmentation germ. 



What other change then occurs ? 



The ovum receives an addition of a layer of clear albuminous 

 material while still in the Fallopian tube, which adds considerably 

 to its bulk. This corresponds to the white of a hen's egg. 



What is segmentation of the vitellus ? 



The next change which occurs in the impregnated egg is the split- 



