294 ON TIJE OVUM OF OSMEUUS EPERLANUS. [^I<\V 4, 



had not been fertilized and were all dead. But they all possessed 

 a kind of membranous appendage, and there were two or three which 

 were suspended from the surface of the stones by means of this 

 membrane, the distal end of which had become attached at the 

 moment of extrusion. In the free eggs no power of adhesion any 

 longer existed. It was ob^^ous enough that the membranous apj)en- 

 dage was the so-called suspensory filament mentioned in the existing 

 literature. But the word filament is a very inappropriate term. 

 The membrane is flexible, and in the form of a hollow truncated 

 cone, the sides of which are thrown into irregular folds ; the narrow 

 end of the cone is continuous with the envelope of the egg; the 

 attachment between the enveloping and the suspensory membrane 

 thus forms a ring on the surface of the former. Examination of the 

 eggs in this condition does not afford evidence of the origin of the 

 suspensory membrane. All that could be seen was that the suspen- 

 sory membrane was dotted all over with pores of considerable size, 

 and that the enveloping membrane was perforated everywhere by 

 finer pores more closely crowded. The enveloping membrane is 

 thus a zona radiata. 



Examination of the eggs freslily pressed from the female gave the 

 complete explanation of all the facts. These eggs were nearly, and 

 some of them quite, mature. They are enclosed in a thick zona 

 radiata, which is differentiated into two layers, the outer of which 

 is somewhat thinner than the internal. In the zona radiata 

 externa the pores are larger and farther apart than in the interna. 

 But the important fact, which I believe no one has previously 

 observed, is that the external zona separates very readily from the 

 internal, and, rupturing at one portion of the ovum, peels off, becom- 

 ing turned inside out in the process, and, remaining attached over a 

 small circular area, forms the suspensory membrane which I have 

 already described. Slight pressure and rolling of the eggs by means 

 of a cover-glass was sufficient to cause the rupture of the external 

 zona, and the two membranes were examined in all stages of separa- 

 tion. Owsjannikow has described the presence of the two layers of 

 the zona radiata in the ovum of Osmerus, and his description agrees 

 with mine ; but the eggs he examined were less mature than those 

 I had to deal with, and it is this fact which prevented him discovering 

 the curious function which the external zona performs. Owsjanni- 

 kow has also described the micropyle in the unripe ova he studied. 

 I was unable to detect the micropyle, but I am inclined to think it 

 exists in the centre of the area over which the suspensory membrane 

 is attached. 



A comparison between the adhesive ova of Osmerus and other 

 adhesive ova can now easily be made. In all adhesive ova the 

 exterior surface of the zona radiata is glutinous ; it adheres to solid 

 objects, and, setting hard after attachment, securely fixes the eggs to 

 one another and to surrounding solids. In some adhesive eggs the 

 external layer of the zona is different in structure from the inner. 

 In the Herring-ovnm there is no distinct differentiation into two 

 layers; in Perca Jluviatilis, as described l)y Owsjannikow, there aie 



