| OCR Text |
Show 294 ON THE OVUM OF OSMERUS KPERLANUS. [May 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 obvious enough that the membranous appendage 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 suspensory 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 freshly 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, becoming 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 separation. 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. Owsjannikow 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-ovum there is no distinct differentiation into two layers; in Percajiuviatilis, as described by Owsjannikow, there aie |
