58 BULLETIN OF THE 



and translucent. With low powers the outer appears as though made 

 up of concentric layers, but with higher powers it is seen to be a 

 homogeneous substance traversed by fine radial canals which are con- 

 tinuous with those passing through the inner layer. At the outer sur- 

 face each of these canals opens out at the base of one of the elevations 

 (Zacken). Calberla regards this whole layer as a secretion from the 

 peripheral layer of the yolk. The proof of it he finds in the conditions 

 of the membranes in nearly ripe and in over ripe eggs. On the former, 

 the boundary between the two layers is sharper and the inner layer is 

 much thicker than on mature eggs ; whereas on the latter all distinction 

 between inner and outer layer has disappeared. 



As soon as the egg comes in contact with the water, the tooth-like pro- 

 jections on the surface of the egg membrane (zona) quickly swell, in con- 

 sequence of which the whole egg appears as if surrounded with a delicate 

 area of hyaline substance. This may well be the cause, he adds, of the 

 stickiness of the surface of the egg. 



It seems to me that there is considerable reason for believing that 

 these external projections described by Calberla correspond to the villi 

 of Lepidosteus, both in function and in position. An examination of his 

 figures (Taf. XXVII.) lends support to this view. I believe also that, 

 when the genesis of the membrane has been studied, it will be found 

 that these "Zacken " are formed before the zona itself. It is true that 



Kupffer und Benecke ('78, pp. 9, 10) find the conditions somewhat 

 different from those recorded by Calberla. They claim that the envelope 

 of the egg consists in both P. Planeri and P. fluviatilis of a double mem- 

 brane (Eihaut), and of a continuous covering of gelatinous material which 

 is replaced at the watch-glass-like elevation of the membrane by a struc- 

 ture known as A. Muller's "Flocke." The inner membrane — which 

 they figure as being much thicker than the outer — contains closely 6et 

 pore-canals, but these they assert positively are not continued into the 

 outer layer. The difference in structure between the two membranes is 

 demonstrable by means of 0.5 per cent hydrochloric acid. The outer 

 membrane swells more, in water than the inner, but not quite uniformly. 

 It appears here and there as though it were restrained by a filament of 

 less easily-swelling substance. And this, they say, is probably the cause 

 of " Calberla's unzutreffende Angabe, dass diese Rindenschicht mit 

 allerlei Erhebungen und Zacken besetzt sei, an deren Basis Poren- 

 canale miindeten." 



But even if Calberla's description is not quite satisfactory, it is evident 

 that this outer envelope is not homogeneous, and that the toothed appear- 



