Osseous fish have almost always a zona radiata, which it seems best to assume to be equivalent to that in Elasmobranchii. Internal to this is a thin membrane, the equivalent, according to Eimer, of the membrane found by the same author within the zona in Reptilia. A membrane equivalent to the thick vitelline membrane of Elasmobranchii would seem to be absent in most instances, though a delicate membrane, external to the zona, has not infrequently been described; Eimer more especially asserts that such a membrane exists in the perch within the peculiar mucous covering of the egg of that fish.

In Petromyzon, a zona radiata appears to be present[420], which is divided in the adult into two layers, both of them perforated. The inner of the two perhaps corresponds with the membrane internal to the zona radiata in other types. In Amphibia the single late formed and radiately striated (Waldeyer) membrane would appear to be a zona radiata. If the suggestion on page 605 turns out to be correct the ova of Mammalia possess both a vitelline membrane and zona radiata. E. van Beneden[421] has, moreover, shewn that they are also provided at a certain period with a delicate membrane within the zona.

The reticulum of the germinal vesicle.—In the course of description of the ovary it has been necessary for me to enter with some detail into the structure of the nucleus, and I have had occasion to figure and describe a reticulum identical with that recently described by so many observers. The very interesting observations of Dr Klein in the last number of this Journal[422] have induced me to say one or two words in defence of some points in my description of the reticulum. Dr Klein says, on page 323, “I have distinctly seen that when nucleoli are present—the instances are fewer than is generally supposed; they are accumulations of the fibrils of the network.” I have no doubt that Klein is correct in asserting that nucleoli are fewer than is generally supposed; and that in many of these instances what are called nucleoli are accumulations, “natural or artificial,” of the fibrils of the network; but I cannot accept the universality of the latter statement, which appears to me most certainly not to hold good in the case of ova, in which nucleoli frequently exist in the absence of the network.

Again, I find that at the point of intersection of two or more fibrils there is, as a rule, a distinct thickening of the matter of the fibrils, and that many of the dots seen are not merely, as Dr Klein would maintain, optical sections of fibrils.

It appears to me probable that both the network and the nucleoli are composed of the same material—what Hertwig calls nuclear substance—and if Dr Klein merely wishes to assert this identity in the passage above quoted, I am at one with him.

Although a more or less distinct network is present in most nuclei (I have found it in almost all embryonic nuclei) it is not universally so. In the nuclei of primitive ova I have no doubt that it is absent, though present in the unmodified nuclei of the germinal epithelium; and it is present only in a very modified form in the nuclei of primitive ova undergoing a transformation into permanent ova. The absence of the reticulum does not, of course, mean that the substance capable of forming a reticulum is absent, but merely that it does not assume a particular arrangement.

One of the most interesting points in Klein's paper, as well as in those of Heitzmann and Eimer, is the demonstration of a connection between the reticulum of the nucleus and fibres in the body of the cell. Such a connection I have not found in ova, but may point out that it appears to exist between the sub-germinal nuclei in Elasmobranchii and the protoplasmic network in the yolk in which they lie. This point is called attention to in my Monograph on Elasmobranch Fishes, page 39[423], where it is stated that “the network in favourable cases may be observed to be in connection with the nuclei just described. Its meshes are finer in the vicinity of the nuclei, and the fibres in some cases appear almost to start from them.” The nuclei in the yolk are knobbed bodies divided by a sponge work of septa into a number of areas each with a nucleolar body.

EXPLANATION OF PLATES 24, 25, 26.

Plate 24.

List of Reference Letters.