GERM CELLS AND THEIR FORMATION 



95 



The various constituent materials of the egg differ, often 

 very considerably, in density, and since they are definitely 

 disposed with reference to the chief egg axis, the eggs tend to 

 assume a definite position with respect to gravity when they 

 are free to move. Usually, in such cases, the yolk is heavier 

 than the protoplasm, and the animal pole is therefore directed 

 upward; this position is reversed occasionally, particularly 

 when the deutoplasm is in the form of oil drops, e.g., Nereis and 

 most Teleosts. 



In some forms the egg cells are naked, without cell coverings 

 or membranes, as in many Ccelenterates and some Molluscs. 

 Or the egg may be naked at first, but soon after becoming free 

 from the parental body it may acquire a thin membrane over 

 its surface, as in Echinoderms. Ordinarily the egg is surrounded 

 by definite membranes of varying nature and origin. The pri- 

 mary egg membrane is the vitelline membrane, 

 or true egg membrane. Typically this is a 

 thin membrane secreted by the superficial 

 protoplasm of the egg and closely applied to 

 its surface (Figs. 42, 45, 46, 47). In most 

 cases it is quite structureless; sometimes it 

 is thicker and may be perforated radially 

 by minute canals or pores, when it is termed FlG - 49. Section 



,, ,. ~ . n , . ,,. through the egg mem- 



the zona radiata. Occasionally the vitelline branes of the 

 membrane may appear double, showing an mobranch - 



inner zona radiata and an outer structureless 

 layer (Figs. 49, 50). In such cases it is 



(Ovarian egg.) From 

 Ziegler, after Balf our. 

 fe, Follicular epithe- 

 lium;^, outer portion 



possible that the membrane is not wholly of the viteffine mn- 



., . J brane (zona pellu- 



vitellme, or it may be the case more fre- cida); yk, surface of 

 quently that the pores originally passed % f : f 

 completely through the membrane and dis- line . membrane (zona 

 appeared from the outer portion of it upon 

 contact with an external medium. 



The vitelline membrane may envelop the egg completely, or 

 there may be left a minute, funnel-shaped perforation through 

 it at the point where the egg was attached or otherwise espe- 

 cially related to the epithelium of the ovary. This aperture 



