THE FOLLICLE SACS OF THE AMPHIBIAN OVARY. 



247 



ments are made up of two membranes which, as a rule, are 

 separated some distance, although occasionally they lie so close 

 together that they appear to be completely united. The outer 

 membrane is single, compact, and of nearly uniform thickness 

 throughout the entire ovary. The inner membrane, on the con- 

 trary, appears in some places to be very similar in structure to the 

 outer one ; in other parts of the ovary it is found to be composed 

 of two thin layers which always lie close together and are con- 

 nected at frequent intervals. Cells with very large nuclei are 

 closely applied to the outer and inner surfaces of these mem- 

 branes and are also present in the spaces between them. (Figs. 

 3-6, 0. W.) 



The eggs develop between the two membranes composing the 

 wall of the ovary. As they increase in size, they press against 

 the inner membrane of the ovarian wall and cause it to project 

 more and more into the cavity of the ovary. Part of the ovarian 



z.i? 



o. w. 



FIG. i. A portion of an ovarian egg showing the manner of its attachment to the 

 wall of the ovary. Z. P. Zona pellucida; O. W-, outer wall of the ovary. Zeiss. 

 obj. A. Oc. 4. 



wall comes, therefore, to form a sac that incloses the egg and 

 supplies it with nourishment as long as it remains in the ovary. 

 When the egg is still quite small, there is formed, close to its 

 outer surface, a true egg-membrane, the " zona pellucida" or 

 " chorion," which is thick and seemingly homogeneous in struc- 

 ture (Fig. 2, Z. /*.). This membrane which is formed, probably, 

 by the numerous follicle cells that are scattered all over the sur- 

 face of the egg, surrounds the egg during its growth and matura- 

 tion periods and is invariably found around all eggs taken from 

 the body cavity. Numerous cells and occasionally blood cor- 

 puscles (Fig. 2, C.} are found between and under the two layers 



