376 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



VI. The Egg Sac 



The egg sac is set apart for the reception of 

 immature ova which have become separated from the 

 ovary. In this organ the ova complete their growth and 

 undergo the first stage of their maturation process. The 

 egg sac is an unpaired structure, and arises as a single, 

 pouch-like outgrowth of the septum between segments 11 

 and 12. It is directed backwards, and in its simplest 

 condition occupies only segment 12. But it soon enlarges 

 considerably, and extends into the segments behind the 

 twelfth. The sperm sac in the course of its development 

 becomes pushed into the egg sac, so that we have here the 

 curious condition of one sac lying actually within the 

 other (PL III, fig. 11, sp.s., ovs.). 



The cavity is, of course, coelomic in origin, and 

 remains undivided throughout, that is, there is no division 

 of its cavity into smaller ones, such as occurs in the sperm 

 sac. The wall of the eg*g sac is thin and composed of a 

 single layer of flattened peritoneal cells, with no muscular 

 elements at all (PL I, fig. 3, ovs.). 



Egg-cells of different ages can be found in the egg 

 sac, and their position in this organ suggests very 

 forcibly that they have all been derived from the ovary. 

 The youngest oocytes are situated in the most anterior 

 segments : that is, in those nearest the ovary, and many 

 of these have few or no yolk granules. The oocytes, 

 when liberated from the ovary, fall into the cavity of 

 segment 11, in which the ovary lies. As the egg sac is 

 an outgrowth of the posterior septum of this segment, 

 it is a very simple matter for the oocytes to find their way 

 from the segment into the egg sac. 



