The segmentation affects only that part of the ovum of animals which is directly ger minal or formative; and it results in the pro. duction of that layer of organised cells, of variable extent, in the centre of which, in a determinate position and direction, the rudi ments of the embryo are first formed. The process of segmentation is, therefore, the pre lude to the formation of the Blastoderma or germinal membrane of Pander.
The extent, therefore, to which seginenta tion affects the yolk differs greatly according to the amount of the yolk-substance which is directly germinal ; that being in some animals the whole, and in others only a fraction of the yolk, in proportion to the part vihich is only indirectly nutritive. In that group of ova, then, to which those of Mammalia belong, and which we have called the small-yolked, the entire yolk, or, at all events, its superficial layer, being directly formative, or being in volved from the first in the production of the Blastoderm, the segmentation is complete, or the process of cleavage affects the whole mass of the finely granular yolk within the 2ona or vitelline membrane. In those ova again, such as we find in the bird among vertebrate, and the cuttlefish among the invertebrate animals, in which the formative yolk has the most limited extent, and consists only of a finely granular disc near the surface of the much larger mass of the cellular nutritive yolk, the segrnentation is confined to that disc alone, and is therefore, in some respects, widely different from that which occurs in 111am malia. In the intermediate group of ova be longing to Batrachia and Osseous Fishes, there are many gradations of transition from the complete to the partial cleavage, so that in some, as the common frog, it is nearly, but not entirely, over the whole yolk ; while in others, as in the salmon or osseous fishes, it does not extend over more than a third of the surface of the yolk.* In the greater number of instances there is recognised in the mass of the whole yolk, or in its germinal part, immediately previous to its undergoing segmentation, a clear simple cell, generally nucleated, which was not be fore perceptible ; to this the name of ern6r,yo cell has been given, in order to distinguish it from the germinal vesicle, from which it has hitherto been believed it is in some measure distinct. In other instances a clear spherule or space only' is observed in the place of the ernbryo-cell, and in a few aniznals no clear part of this natttre has yet been detected. The division of the embryo-cell accompanies, or rather immediately precedes, that of the germ-yolk, so that each mass formed by the cleavage, grooving, or segmentation, as the case may be, contains as its nucleus or centre an embryo-cell, or clear spherule of its own, descended from the first cell or spherule of the same description.
The process of seg,rnentation, whether it involves the whole ovum, or is limited to a larger or snialler disc of the yolk, proceeds in most animals with a certain degree of geo metric regularity', so that the number of germ yolk segments are successively multiplied so as to be in the numbers 2, 4, 8, 16, 32, 64, &c., untii by the ultimate division a vast number of small globular masses are formed, which occupy principally the surface of the yolk over all its germinal portion.* The last result of the segmentation is the production of the blastoderma or germinal membrane in which, by other changes, the rudiments of the embryo subsequently make their appearance. According to most ovo logists, the last globules formed by segmenta tion are the nucleated organised cells im mediately constituting the blastoderma. A different view of the process, however, in Mammalia, has been taken by Bischoff, very decidedly set forth in his two most recent works on the development of the guinea-pig and the deer ; according to which the last resulting spherules formed by segmentation are not true cells, and that previous to the formation of the blastodermic cells, the yolk germ falls completely into an amorphous or homogeneous finely granular substance, out of which, secondarily, the blastodermic cells are produced by a process of cytogenesis. It seems probable that, in the different classes of animals, there may be considerable variety in the degree of perfection in organisation or ad vance in cell-structure to which the segments of the yolk have attained at the period when the development of the embryo begins to ma nifest itself. But in the higher animals at least the weight of evidence appears to me in favour of the view that the process of segmentation results directly in the formation of blastodermic cells. The fact now established by the obser vations of Reichert in Entozoa, in 1811, of Ransom in osseous fishes, and more particu larly those of Remak in Batrachia, that a de licate rnembrane is formed over the surface of each of the segments as they appear, and that the last and smallest segments possess a deli cate membranous envelope, appear to show that, in these animals, each segment has the structure of an organised cell, and is very si milar to, if not identical with, those of the blastodermic larnina.