The works of Francis Maitland Balfour, Volume 1 (of 4)

by Francis M. Balfour

The nests are situated for the most part near the surface of the germinal epithelium. The smaller ones are frequently spherical, but the larger are irregular in form. The former are about 0.05 mm. in diameter; the latter reach 0.1 mm. Scattered generally, and especially in the deeper layers, and at the edges of the germinal epithelium, are still unmodified or only slightly modified primitive ova. These unmodified primitive ova are aggregated in masses, but in these masses the outlines of each ovum, though perhaps less clear than in the earlier period, are still distinct.

When the embryo reaches a length of seven centimètres, and even in still younger embryos, further changes are observable. In the first place many of the modified nuclei acquire fresh characters, and it becomes necessary to divide the modified nuclei into two categories. In both of these the outer boundary of the nucleus is formed by a very delicate membrane, the space within which is perfectly clear except for the granular body. In the variety which now appears in considerable numbers the granular body has an irregular star-like form. The rays of the star are formed of fibres frequently knobbed at their extremities, and the centre of the star usually occupies an eccentric position. Typical examples of this form of modified nucleus, which may be spoken of as the stellate variety, are represented on Pl. 25, fig. 17; between it and the older granular variety there is an infinite series of gradations, many of which are represented on Pl. 24, figs. 12, 14, 15, 16. Certain of the stellate nuclei exhibit two centres instead of one, and in some cases, like that represented on Pl. 25, fig. 19, the stellate body of two nuclei is found united. Both of these forms are possibly modifications of the spindle-like form assumed by nuclei in the act of dividing, and may be used in proving that the nests increase in size by the division of the contained nuclei. In addition to the normal primitive ova, a few of which are still present, there are to be found, chiefly in the deeper layers of the germinal epithelium, larger ova differing considerably from the primitive ova. They form the permanent ova (Pl. 24, fig. 3, o). Their average diameter is 0.04 mm., compared with 0.03 mm., the diameter of original primitive ova. The protoplasm of which they are composed is granular, but at first a membrane can hardly be distinguished around them; their nucleus is relatively large, 0.02 - 0.027 mm. in diameter. It presents the characters ascribed by Eimer[375], and many other recent authors[376], to typical nuclei (vide Pl. 24, fig. 3, and Pl. 24, 25, figs. 13, 14, 15, 16, 17, 18). It is bounded by a distinct membrane, within which is a more or less central nucleolus from which a number of radial fibres which stain very deeply pass to the surface; here they form immediately internal to the membrane a network with granules at the nodal points. In some instances the regularity of the arrangement of these fibres is very great, in other instances two central nucleoli are present, in which case the regularity is considerably interfered with. The points in which the youngest permanent ova differ from the primitive may be summed up as follows:—

(1) The permanent ova are larger, the smallest of them being larger than the average primitive ova in the proportion of four to three. (2) They have less protoplasm as compared to the size of the nucleus. (3) Their protoplasm is granular instead of being clear. (4) Their nucleus is clear with exception of a network of fibres instead of being granular as in the primitive ova. It thus appears that the primitive ova and permanent ova are very different in constitution, though genetically related in a way to be directly narrated.

The formation of permanent ova is at its height in embryos of about seven centimètres or slightly larger. The nests at this stage are for the most part of a very considerable size and contain a large number of nuclei, which have probably, as before insisted, originated from a division of the smaller number of nuclei present in the nests at an earlier stage. Figs. 14-18 are representations of nests at this period. The diameter of the nuclei is, on the whole, slightly greater than at an earlier stage. A series of measurements gave the following results:—

0.016 mm.
0.016 mm.
0.018 mm.
0.020 mm.
0.020 mm.

Both varieties of modified nuclei are common enough, though the stellate variety predominates. The nuclei are sometimes in very close contact, and sometimes separated by protoplasm, which in many instances is very slightly granular. In a large number of the nests nothing further is apparent than what has just been described, but in a very considerable number one or more nuclei are present, which exhibit a transitional character between the ordinary stellate nuclei of my second category, and the nuclei of permanent ova as above described; and in these nests the formation of permanent ova is taking place. Permanent ova in the act of development are indicated in my figures by the letters do. Many of the intermediate nuclei are more definitely surrounded by granular protoplasm than the other nuclei of the nests, and accordingly have their outlines more sharply defined. Between nuclei of this kind, and others as large as those of the permanent ova, there are numerous transitional forms. The larger ones frequently lie in a mass of granular protoplasm projecting from the nest, and only united with it by a neck (Pl. 24, figs. 14 and 16). For prominences of this kind to become independent ova, it is only necessary for the neck to become broken through. Nests in which such changes are taking place present various characters. In some cases several nuclei belonging to a nest appear to be undergoing conversion into permanent ova at the same time. Such a case is figured on Pl. 25, figs. 17 and 18. In these cases the amount of granular protoplasm in the nest and around each freshly formed ovum is small. In the more usual cases only one or two permanent ova at the utmost are formed at the same time, and in these instances a considerable amount of granular protoplasm is present around the nucleus of the developing permanent ovum. In such instances it frequently happens several of the nuclei not undergoing conversion appear to be in the process of absorption, and give to the part of the nest in which they are contained a very hazy and indistinct aspect (Pl. 24, fig. 15). Their appearance leads me to adopt the view that while some of the nuclei of each nest are converted into the nuclei of the permanent ova, others break down and are used as the pabulum, at the expense of which the protoplasm of the young ovum grows.

It should, however, be stated, that after the outlines of the permanent ova have become definitely established, I have only observed in a single instance the inclusion of a nucleus within an ovum (Pl. 25, fig. 24). In many instances normal nuclei of the germinal epithelium may be so observed within the ovum.

The nuclei which are becoming converted into the nuclei of permanent ova gradually increase in size. The following table gives the diameter of four such nuclei:—

0.022 mm.
0.022 mm.
0.024 mm.
0.032 mm.

These figures should be compared with those of the table on page [564].