DEVELOPMENT OF THE IMPREGNATED EGG. 617 



afterward coming in contact with the remainder of the germinative 

 vesicle, the two unite into a single mass. This new product, made 

 up partly of the germinative vesicle and partly of the spermatozoon, 

 then assumes the position and appearance of a central nucleus. It is 

 regarded as the point of origin for all subsequent changes in the im- 

 pregnated egg. 



Deposit of Albuminous Layers in the Fallopian Tube. As the 

 impregnated egg passes down the Fallopian tube, it becomes covered 

 with an albuminous secretion. In birds, this secretion is deposited in 

 layers round the vitellus, forming the so-called " white of egg." In 

 reptiles, it is also poured out in considerable quantity, and serves for 

 the nourishment of the egg during its development. In mammalians, 

 a similar secretion is supplied in smaller quantity, but sufficiently 

 abundant in proportion to the size of the egg in the earlier stages 

 of its growth, before it has established a connection with the lining 

 membrane of the uterus. 



Segmentation of the Vitellus. A remarkable change now takes place 

 in the impregnated egg, by which its structure is definitely altered. 

 This is known as the division, or " segmentation " of the vitellus. Its 

 globular mass Is marked by a circular furrow, which gradually deepens 

 until it divides the vitellus into two nearly equal halves or hemispheres. 

 Each hemisphere is then found to contain a nucleus, similar to that 

 which previously occupied the centre of the impregnated vitellus (Fig. 



SEGMENTATION OF THE VITELT.U*, in the impregnated egg of the rabbit. (Coste.) 



It6, A). Almost at the same time a second furrow, at right angles 

 with the first, penetrates the vitellus in a similar way, and cuts it in a 

 transverse direction. The vitellus is thus divided into four equal por- 

 tions, of a rounded form, lying for the most part in contact with each 

 other and embraced by the vitelline membrane (Fig. 176, B). The 



