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Show 1886.] OVUM OF LEPIDOSIREN. 273 Concerning the origin of the ova I have no observations to offer. I shall therefore at once proceed to the discussion of the different parts of the ovum and its follicular epithelium. II. Egg-membranes. The question of the origin of the different egg-membranes and their homologies throughout the Vertebrate series has always been one of interest; but the opinions held with regard to these points differ very much. Balfour considers it tolerably certain l that in most Vertebrata there are two egg-membranes present-(1) an outer structureless vitelline membrane, and (2) an inner perpendicularly striate zona radiata, both of which are formed as a differentiation of the egg-protoplasm. Of these two membranes the vitelline isobviously the first to appear ; the zona radiata subsequently makes its appearance, but is reabsorbed into the ovum some time before the latter is mature ; the vitelline membrane persists for a longer period, but eventually disappears. In the youngest ova of Protopterus I could detect no trace of any membrane ; as development proceeds a very delicate homogeneous membrane encircles the ovum. This membrane probably corresponds to the vitelline membrane, but I have no exact observations to offer on its origin; judging from analogy it is a product of the egg-protoplasm, which at this stage (Plate X X I X. fig. 8) is of uniform granular appearance. At the same time the membrane was not at all firmly adherent to the ovum, but seemed to have a much more intimate relation to the follicular epithelium; in sections that were less perfect than others this membrane was detached from the surface of the ovum. In more mature ova-of the stage represented in Plate XXVIII. fig. 1-there was present, in addition to the vitelline membrane, a thick membrane lying beneath it, and evidently formed as a specialization of the peripheral layer of the ovum. For the most part this membrane was as distinct from the subjacent protoplasm of the ovum as is shown in Plate X X I X . fig. 2. Here and there it appeared to pass gradually and without any break into the substance of the protoplasm. This membrane shows radial striations (Plate XXIX. fig. 2), and appears to m e in all probability to correspond to the zona radiata of other Vertebrata. The extreme thinness of the vitelline membrane as compared with the zona radiata is illustrated in Plate X X I X . fig. 2. In this stage there was no trace of any formation of yolk except that the egg-protoplasm has taken on a reticulate arrangement (see Plate X X I X . fig. 2) in preparation for the yolk-formation soon to commence. In the next stage, in which yolk-formation has already commenced, the conditions of the egg-membranes are slightly different, the difference principally relating to the outer vitelline membrane. The vitelline membrane in this stage is very markedly thicker than in the previous stage ; it is also much more intimately connected with the follicular epithelium than with the zona radiata. The latter 1 Comp. Embr. vol. i. p. 50. |