344 LEWIS V. HEILBRUNN. 



used. As it emerges from the ovary the unfertilized egg is 

 already surrounded by a broad band of gelatinous material, 

 the chorion. Directly underneath the chorion lies the peripheral 

 surface of the egg. After fertilization, a membrane is clearly 

 seen beneath the chorion and some little distance from the egg; 

 this is the vitelline (or so-called fertilization membrane). The 

 presence of this membrane in the unfertilized egg has often been 

 disputed. Hertwig 1 was of the opinion that a definite preformed 

 membrane exists. Such a condition seemed to Fol 2 to preclude 

 the penetration of the sperm, a fact which he had for the first 

 time observed. Accordingly, he states that the egg is not sur- 

 rounded by a membrane, but by a "hyaline layer." Upon 

 fertilization, he observes that this layer becomes lifted from the 

 egg and forms the vitelline membrane. Herbst 3 first showed 

 that this "hyaline layer" of Fol was a true membrane in that it 

 possessed rigidity and was distinct from the underlying cyto- 

 plasm. He found on pressing unfertilized eggs under a cover 

 glass, that the protoplasm of the egg flowed out, leaving behind 

 the membrane. 4 Schiicking 5 isolated the membrane by cutting 

 the eggs. Finally Kite 6 has been able to dissect away the mem- 

 brane with the aid of a Barber pipette. No doubt the difficulty 

 in observing the membrane is due to the fact that its refractive 

 index is almost identical with that of sea-water. If the refractive 

 index of sea-water is increased, e. g., by the addition of a protein, 

 the vitelline membrane becomes clearly visible. The observa- 

 tion of this membrane in such liquids of greater refractive index 

 has led several observers to suppose that it is "formed" there. 

 Since membranes "formed" in this way are penetrable to sperm, 

 Loeb 7 has proposed to call them "pseudomembranes." 



In as much as the presence of a preformed membrane has been 

 demonstrated by a number of observers, it is incorrect to speak 

 of the "formation" of a vitelline membrane at fertilization. 



1 O. Hertwig, Morph. Jahrb., I., 347 (1875). 



2 H. Fol, Ann. d. set. phys. el not., LVIII., 439 (1877); *'& Mem. d. I. Soc. d. 

 phys. et d'hisl. nat. d. Geneve, XXVI. (1879). 



3 C. Herbst, Biol. Cenlralbl., XIII., 14 (1893). 



4 This fact had already been noted by Hertwig (loc. cit.). 

 6 Schiicking, Arch. f. d. ges. PhysioL, XCVII., 68 (1903). 



6 G. L. Kite, Science, N. S., XXXVI., 562 (1912). 



7 J. Loeb, Arch. f. Ent. Mech., XXVI., 82 (1908). 



