722 



DEVELOPMENT OF THE IMPREGNATED EGG. 



Fig. 240. 



sufficiently abundant, in proportion to the size of the mammalian -egg; 



and it serves for the supply of organic material in the earlier stages of 



development, before the egg has established its connection with the 



uterine mucous membrane. 



Segmentation of the Vitellus. A very important change now takes 



place in the impregnated egg, which is known as the division, or seg- 

 mentation, of the vitellus. A furrow 

 shows itself, running round the globular 

 mass of the vitellus in a vertical direction, 

 which gradually deepens until it has di- 

 vided the vitellus into two separate halves 

 or hemispheres (Fig. 240, a.) Almost at 

 the same time another furrow, running at 

 right angles with the first, penetrates the 

 substance of the vitellus, and cuts it in a 

 transverse direction. The vitellus is thus 

 divided into four equal portions (Fig. 240, 

 6), the edges and angles of which are 

 rounded off, and which are still contained 

 in the cavity of the vitelline membrane. 

 The spaces between them and the internal 

 surface of the vitelline membrane are oc- 

 cupied by a transparent fluid. 



The process thus commenced goes on 

 by a successive formation of furrows and 

 sections, in various directions. The four 

 vitelline segments already produced are 

 subdivided into sixteen, the sixteen into 

 sixty-four, and so on ; until the whole vi- 

 tellus is converted into a mulberry-shaped 

 mass of minute, nearly spherical bodies, 

 called the "vitelline spheres." (Fig. 240, c.) 

 The vitelline spheres have a somewhat 

 firmer consistency than the original sub- 

 stance of the vitellus ; and this consistency 

 appears to increase as they multiply in 

 numbers and diminish in size. At last 

 they become so abundant as to be closely 

 crowded together and compressed into po- 

 lygonal forms. (Fig. 240, d.} They have 

 by this time been converted into a layer of 

 cells, surrounding the original central cav- 

 ity of the egg, and themselves enveloped 

 by the vitelline membrane. 

 The segmentation of the vitellus constitutes the primary act in the 



development of the impregnated egg. It is this remarkable process 



which is the sign that fecundation has taken place, and that the forma- 



SEGMENTATION OF THE 

 VIT KLLUS. 



