PROMORPHOLOGICAL RELATIONS OF CLEAVAGE The cleavage of the ovum has thus far been considered merely as a problem of cell-division. We have now to regard it in a far more interesting and suggestive aspect ; namely, in its morphological relations to the body to which it gives rise. From what has been said thus far it might be supposed that the egg simply splits up into indifferent cells which, to use the phrase of Pfliiger, have no more definite relation to the structure of the adult body than have snow-flakes to the avalanche to which they contribute. Such a conclusion would be totally erroneous. It is a remarkable fact that in a very large number of cases a precise relation exists between the cleavage-products and the adult parts to which they give rise ; and this relation may often be traced back to the beginning of development, so that from the first division onwards we are able to predict the exact future of every individual cell. In this regard the cleavage of the ovum often goes forward with a wonderful clock-like precision, giving the impression of a strictly ordered series in which every division plays a definite role and has a fixed relation to all that precedes and follows it.
But more than this, the apparent predetermination of the embryo may often be traced still further back to the regions of the undivided and even unfertilized ovum. The egg, therefore, may exhibit a distinct promorphology ; and the morphological aspect of cleavage must be considered in relation to the promorphology of the ovum of which it is an expression.
the embryo is definitely related to it. The great embryologist pointed out, further, that the early cleavage-planes also are definitely related to it, the first two passing through it in two meridians intersecting each other at a right angle, while the third is transverse to it, and is hence equatorial.' Remak afterwards recognized the fact 2 that the larger cells of the lower hemisphere represent, broadly speaking, the " vegetative layer " of von Baer, i.e. the inner germlayer or entoblast, from which the alimentary organs arise ; while the smaller cells of the upper hemisphere represent the "animal layer," outer germ-layer or ectoblast from which arise the epidermis, the nervous system, and the sense-organs. This fact, afterwards confirmed in a very large number of animals, led to the designation of the two poles as animal and vegetative, formative and nutritive, or protoplasmic and deutoplasmic, the latter terms referring to the fact that the nutritive deutoplasm is mainly stored in the lower hemisphere, and that development is therefore more active in the upper. The polarity of the ovum is accentuated by other correlated phenomena. In every case where an egg-axis can be determined by the
accumulation of deutoplasm in the lower hemisphere the egg-nucleus sooner or later lies eccentrically in the upper hemisphere, and the polar bodies are formed at the upper pole. Even in cases where the deutoplasm is equally distributed or is wanting — if there really be such cases — an egg-axis is still determined by the eccentricity of the nucleus and the corresponding point at which the polar bodies are formed.
In vastly the greater number of cases the polarity of the ovum has a definite promorphological significance ; for the egg-axis shows a definite and constant relation to the axes of the adult body. This relation is, it is true, somewhat variable in different animals, yet the evidence indicates that as a rule it is constant in a given species. It is a very general rule that the upper pole, as marked by the position of the polar bodies, lies in the median plane at a point which is afterwards found to lie at or near the anterior end. Throughout the annelids and mollusks, for example, the upper pole is the point at which the cerebral ganglia are afterwards formed ; and these organs lie in the adult on the dorsal side near the anterior extremity. This relation holds true for many of the Bilateralia, though the primitive relation is often disguised by asymmetrical growth in the later stages, such as occur in echinoderms. It is not, however, a universal rule. The recent observations of Castle ('96), which are in accordance with the earlier work of Seeliger, show that in the tunicate Ciona the usual relation is reversed, the polar bodies being formed at the vegetative (i.e. deutoplasmic) pole, which afterwards becomes the ventral side of the larva. In Ascaris Boveri's observations seem to show that the position of the polar bodies has no constant relation to the adult axes, and Hacker describes a similar variability in the copepods (Fig. 130). My own observations on the echinoderm-egg indicate that here also the primitive egg-axis has an entirely inconstant and casual relation to the gastrula-axis. It may perhaps still be possible to show that these exceptions are only apparent, and the principle involved is too important to be accepted without further proof. As the facts now stand, however, they seem to admit of no other conclusion than that the relation of the primitive egg-axis to the adult axes is not absolutely constant, and may in particular cases be variable. To admit this is to grant that this relation is not of a fundamental character, and that the axes of the adult are not predetermined from the beginning, but are established in the egg in the course of development.