252 ALICE THING 



2. The zona pellucida varies in thickness from 1^ to 17/x ac- 

 cording to the stage of development of the egg. Beginning with 

 a stage where it is on an average 3/x thick two different laj^ers 

 appear, the outer denser and thicker and the inner narrower, 

 clearer and striated. In the course of development the outer 

 layer differentiates, grows and extends to a greater degree than 

 the inner. 



3., The zona pellucida during its growth is always formed by 

 two or three different elements : 



a. The fundamental homogeneous substance filling up the 

 spaces between 



h. A system of numerous canals or tubules which inclose 



c. Filaments or prolongations of the epithelial cells which are 

 connected with the surface of the yolk. The fundamental sub- 

 stance of the zona pellucida is more abundant and dense in the 

 outer layer than in the inner. 



4. The fundamental substance of the zona pellucida is de- 

 veloped as a cuticular element by the terminal bars or primary 

 netw^ork, that is by a definite special intercellular cement possess- 

 ing the property of extension over the free surface of the epithelial 

 cells and forming connections there with the delicate secondary 

 network apparently produced directly by the superficial cyto- 

 plasm of the epithelial cells. The secondary network seems able 

 to give rise at its surface to a cement similar to that resulting 

 from the activity of the terminal bars. This superficial cuticular 

 network gradually becomes thicker and by the development of 

 fresh cuticular material builds up the entire fundamental sub- 

 stance of the zona pellucida. The prolongations of the epithelial 

 cells, at first short, traverse the zona pellucida and become longer 

 as this increases in thickness. Enclosed in canals, the pro- 

 longations reach the surface of the yolk to end in knob-like 

 enlargements. 



5. The structure of the zona pellucida just described presents 

 a condition most favorable for the conveyance of nutritive mate- 

 rial from the epithelial area in contact with the maternal capillaries 

 to the actively growing and extending yolk. 



