CELL-DIVISION AND DEVELOPMENT Since the early work of Kolliker and Remak it has been recognized that the cleavage or segmentation of the ovum, with which the development of all higher animals begins, is nothing other than a rapid series of mitotic cell-divisions by which the egg splits up into the elements of the tissues. This process is merely a continuation of that by which the germ-cell arose in the parental body. A long pause, however, intervenes during the latter period of its ovarian life, during which no divisions take place. Throughout this period the egg leads, on the whole, a somewhat passive existence, devoting itself especially to the storage of potential energy to be used during the intense activity that is to come. Its power of division remains dormant until the period of full maturity approaches. The entrance of the spermatozoon, bringing with it a new centrosome, arouses in the egg a new phase of activity. Its power of division, which may have lain dormant for months or years, is suddenly raised to the highest pitch of intensity, and in a very short time it gives rise by division to a myriad of descendants which are ultimately differentiated into the elements of the tissues.
The divisions of the egg during cleavage are exactly comparable with those of tissue-cells, and all of the essential phenomena of mitosis are of the same general character in both. But for two reasons the cleavage of the egg possesses a higher interest than any other case of cell-division. First, the egg-cell gives rise by division not only to cells like itself, as is the case with most tissue-cells, but also to many other kinds of cells. The operation of cleavage is therefore immediately connected with the process of differentiation, which is the most fundamental phenomenon in development. Second, definite relations may often be traced between the planes of division and the structural axes of the adult body, and these relations are sometimes so clearly marked and appear so early that with the very first cleavage the position in which the embryo will finally appear in the egg may be exactly predicted. Such " promorphological " relations of the segmenting egg possess a very high interest in their bearing on the theory of germinal localization and on account of the light which they throw on the conditions of the formative process.
The present chapter is in the main a prelude to that which follows, its purpose being to sketch some of the external features of early development regarded as particular expressions of the general laws of cell-division. For this purpose we may consider the cleavage of the ovum under two heads, namely : The Geometrical Relations of Cleavage-forms, with reference to the general laws of cell-division.