NO. 1783. 'NORTH AMERICAN LERN^OPODIDM— WILSON. 195 



method obtains in the Ergasilidse, whose eggs mature successively 

 and are pushed out into the external strings in bunches, a few at a 

 time. 



STRUCTURE OP THE EGG. 



Beneden states that he was never able to distinguish any mem- 

 brane around the ovarian egg of Anchorella or Lernxopoda, but that 

 the greater density of the protoplasm near the periphery of the egg 

 preserved its spheroidal form. 



In Achtheres amUoplitis a vitelline membrane is distinctly visible 

 around the enlarged oocytes, even before they separate from ihe 

 cell filaments (fig. 2). This membrane is very thin and structure- 

 less; inside of it the entire substance of the egg is made up of yolk 

 globules evenly and universally distributed tliroughout a matrix of 

 fine protoplasm. These globules difTer only slightly in size and 

 shape, being usually somewhat ovate or ellipsoidal and occasionally 

 flattened by contact with one another. They are homogeneous in 

 structure, but some of them stain more deeply than others. 



Scattered about through the egg are numerous large vacuoles, 

 varying from one to four times the diameter of the yolk globules. 

 In the sections they are entirely empty, save for the intrusion of 

 an occasional yolk globule, but in the living egg are probably filled 

 with a liquid which has been withdrawn by the preservatives. The 

 vacuoles nearest the periphery are usually the smallest, and they 

 increase in size toward the center of the egg. The egg nucleus is 

 about the size of the largest vacuoles and is slightly eccentric, away 

 from the point of attachment of the cell filament. It is approxi- 

 mately spherical and is surrounded by a well-defined membrane. It 

 contains a single spherical nucleolus of small size and situated close 

 to the periphery, and numerous chromatin granules, also gathered 

 near the outer surface of the nucleus. The cytoplasm just inside 

 the vitelline membrane is denser than elsewhere, and particularly 

 in the younger oocytes takes a bluish tinge in hsematoxylin. In the 

 matured oocytes this is scarcely visible, but the increased density 

 and the fineness of the granules are very distinct. 



As the eggs pass down through the oviduct they are each fertilized 

 at the opening of the sperm receptacle and are covered with a layer 

 of the cement substance at the opening of the duct of the cement 

 gland just before they pass out into the external sacks. This cement 

 forms a thick outer egg membrane or shell, separated a little from 

 the vitelline membrane and stiffening into a strong and tough cov- 



ermg. 



Beneden called the vitelline membrane the chorion and this shell 

 membrane the exochorion. The egg sack itself is made of the same 

 material, only thicker and tougher. Inside of it the eggs lie freely 



