180 WILLIAM WALLACE. 



applying gentle pressure to the egg tlie " zona " ruptures, 

 and the vitellus tightly clothed with the line homogeneous 

 vitelline membrane protrudes through the fissure thus pro- 

 duced. 



(2) Zona radiata, 1 



,o\ -ir-n 1 /r • 1 J. \ ^ produced by the ovum, 



(d) Villous layer (Lepidosteus); J ^ -^ 



(4) Miiller's capsular membrane in the perch and pike — a 

 product of the granulosa or follicular epithelium. 

 Eigenmann gives the following classification : 

 (1) Eggs with single membrane. The zona radiata : a. 

 Zona a single layer of uniform structure ; b. Zona differen- 

 tiated into an outer and inner layer. 



\j (2) Eggs with a zona radiata and a thin homogeneous 

 outer layer. The outer membrane may bear appendages. 



(3) Eggs with a zona and a thick outer layer produced by 

 secretion of the granulosa cells. 



In addition to all these a " zonoid " membrane has been 

 described. It was discovered in the eggs of teleosteans by His, 

 and subsequently observed by Emery (1880), Scharff (1887), 

 Eigenmann (1890), and others. It underlies the zona, and 

 according to Scharff, who found it in the immature ovarian 

 ova of the gurnard, is " semi-fluid, usually devoid of granules, 

 and stains only slightly." In ripe ova it disappears entirely. 

 The zona is formed before the zonoid layer. In Blennius 

 pholis there is no zonoid layer (?). 



In Elasmobranchs the number of egg membranes appears 

 from the observations of Balfour (1878), Giacomini (189G), 

 and others to be constant, viz. two. These are an outer 

 homogeneous one, named by Balfour the " vitelline mem- 

 brane " — though quite distinct from the true vitelline 

 membrane above mentioned, — and an inner radially striated 

 one, the " zona radiata.'^ Beddard observed two apparently 

 homologous membranes in the ova of Dipnoi. Emery (1880) 

 draws attention to the similarity of the zonoid layer in 

 Fierasfer to the zona radiata of Balfour. tScharff, again, 

 identifies the zonoid layer in the gurnard with Beddard's 

 zona radiata. 



