188 WILLIAM WALLACE. 



represent most accurately what takes place at the surface of 

 the egg. His figs. 42, 43 speak for themselves, but his 

 concise description may advantageously be quoted here. He 

 says ('La Cellule/ xvii, p. 47), "La membrane de I'oeuf est 

 d'abord simple, et formee d'une seul couclie visible, ;\ laquelle 

 aboutissent les trabecules du reticulum cytoplasmatique. 

 Cette membrane, tres fine au debut, s'epaissit dans le cours 

 du developpement de I'oeuf ; de nouvelles couches s'ajoutent 

 progressivement de dehors en dedans. A cet effet les 

 trabecules se regularisent sous la membrane; les unes s'or- 

 donnent radialement les autres s'orientent perpendiculaire- 

 ment a, celles-ci de facon a former une strie concentrique 

 bien reguliere. Cette strie d'abord assez eloignee des 

 couches preexistantes, s'en rapproche de plus en plus et 

 devient partu integrante de la membrane." 



Turning now to Elasmobranchs, we find in Spin ax and 

 Chimtera apparently the same two membranes that we met 

 with in Zoarces, etc., viz. the zona and the zonoid layer. I 

 refer, of course, to the immature eggs, for, as Balfour (1878) 

 has shown, the membranes (in Scyllium and Raia) are 

 absorbed before the ovum quits the follicle. It happens that 

 in fig. 19, which represents a section at the surface of an 

 immature egg of Spin ax, the zonoid layer is not clearly seen. 

 The hardening action of reagents and the subsequent strain 

 of osmosis on the hardened structures have caused the vitellus 

 to separate from the zonoid layer, which has in turn greatly 

 contracted, so as to be scarcely visible at this particular 

 point. In other parts, however, of the same egg the zonoid 

 layer can be distinctly seen ; it is perfectly constant in 

 Spin ax and in Chimasra (figs. 29, 30), and has the same 

 properties as in Zoarces. 



The portion of the section drawn in fig. 19 was specially 

 selected because it shows the relation of the cells of the 

 follicular epithelium to the egg membrane. As a result of 

 the contracting effect of reagents, the follicular epithelium is 

 raised up from the egg membrane, and its cells are seen to 

 be pi-ovided with delicate processes, the distal ends of which 



