ZONA PELLUCIDA IN TURTLE EGGS 245 



show clearly in the photographs. Some preparations fixed in 

 Bouin and stained by Mallory's connective tissue method, show 

 this substance very clearly colored by aniline blue. The inter- 

 cellular substance early undergoes a change of constitution and 

 becomes transformed, at the level of the surface of the cells, into 

 the special cement known as the terminal bars (Schiifer '12, p. 86, 

 Stohr '98, p. 68) . It is well known that sections cut perpendicular 

 to the plane of the surface of the epithelial cells show in well fixed 

 and stained preparations a continuous dark line representing 

 the lateral surfaces of the terminal bars sometimes thickened 

 noticeably at points marking the limits of two adjacent cells. 

 In other portions of the sections this line may not be seen but 

 cross sections of the bars appear as dark round spots. The for- 

 mer picture is represented in the turtle's eggs in figure 2, t.b. 

 The lateral surfaces of the terminal bars of adjacent cells form a 

 rather thick distinct boundary line between themselves and the 

 oocyte thus marking the beginning of the zona pelucida. 



Cytoplasmic bridges of various sorts connect the cells with 

 one another (Fischer '05, Paladino '90). Filamentous and thin 

 or short and coarse, they traverse the intercellular spaces 

 and retain their identity for considerable distances within the 

 cell cytoplasm where they finally mingle with the denser portions 

 encircling the large nuclei (fig. 1 Lb., s.b.). A dense opaque mass, 

 the attraction sphere, is closely attached to each nucleus usually 

 either on that face which is nearest the surface of the cell or at 

 one side (figs. 2, 4, a.s.). Often such clearness is obtained through 

 successful fixation or through the thinness of the section as to 

 determine the character of the sphere. It is composed of three 

 elements, a small granule (or sometimes two) the central cor- 

 puscle in the center or slightly to the side of an oval or circular 

 clear field, the medullary layer marked off from the mass by a 

 distinctly larger, more opaque zone, the cortical layer (Van Bene- 

 den). Loosely interwoven filaments extend out from the dense 

 attraction sphere to the clear exoplasm at the periphery of the 

 cells thus forming a delicate network. 



