CHAPTER VI. 



DEVELOPMENT OF THE IMPREGNATED EGG SEGMEN- 

 TATION OF THE VITELLUS BLASTODERM- 

 FORMATION OF ORGANS IN THE FROG. 



fTlHE unimpregnated egg has a certain period of growth within the 

 -L Graafian follicle, during which it increases in size from the insig- 

 nificant dimensions of its earlier formation to those of its maturity as 

 an ovarian egg. The vitellus, at first transparent and colorless, be- 

 comes granular and opaque, at the same time that its mass is enlarged 

 by the deposit of new elements; and in birds and reptiles it also 

 acquires a distinctive hue, generally orange or yellow. These modifi- 

 cations are the result of its spontaneous growth, the materials for 

 which are supplied from the ovarian tissues. At its completion, when 

 the egg is ready to be discharged from the ovary, it consists of the 

 fully formed vitellus, enclosed in a vitelline membrane, and containing, 

 imbedded in its substance, the germinative vesicle with the germina- 

 tive spot. 



Thus constituted, the egg leaves the ovary on the rupture of the 

 ovarian follicle, and enters the Fallopian tube. Here, if coition have 

 taken place, it meets with the spermatozoa, and by their contact and 

 penetration it is made ready for the production of the embryo. It 

 is consequently transformed, by impregnation, from a barren offshoot 

 of the ovarian tissue into a new body, in which the male and female 

 elements are united, and which possesses a capacity for further de- 

 velopment. 



Immediate Effects of Impregnation. The first change in the egg, 

 consequent on impregnation, is the disappearance of the germinative 

 vesicle. This feature, always very distinct in the ovarian egg, becomes 

 imperceptible after its contact with the spermatic fluid in the Fallopian 

 tube ; and its place is subsequently taken by a new formation, which 

 is designated as the "nucleus of the impregnated egg." The details 

 of this substitution have not been fully ascertained ; but its important 

 characters, so far as yet known, are mainly as follows.* The germi- 

 native vesicle leaves its position within the vitelline mass and approxi- 

 mates the surface, losing at the same time a portion of its substance, 

 becoming smaller in size and elongated in form. On the other hand, 

 a spermatozoon, which has penetrated into the vitelline sac, becomes 

 also changed by the disappearance of its filamentous portion; and 



* Kolliker, Embryologie. Paris, 1879, p. 55. 



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