Elements of Comparative Anatomy (1878) Elements of Comparative Anatomy elementsofcompar78gege Year: 1878 0 â 9^ 5> ⢠* Ls 1*' Figs. 2-5. Various stages of the so-called cleavage process (Division of the egg). the egg-cell now begins to divide. Two cells thus arise, which are either like one another, or differ from one another, in size or in constitution. In both cases something fresh has arisen from the egg-cell, and in both there is a differentiation, for two parts have arisen from it. Four, eight, sixteen cells, and so on, are formed by continued division, although of course not alwa
Elements of Comparative Anatomy (1878) Elements of Comparative Anatomy elementsofcompar78gege Year: 1878 0 â 9^ 5> ⢠* Ls 1*' Figs. 2-5. Various stages of the so-called cleavage process (Division of the egg). the egg-cell now begins to divide. Two cells thus arise, which are either like one another, or differ from one another, in size or in constitution. In both cases something fresh has arisen from the egg-cell, and in both there is a differentiation, for two parts have arisen from it. Four, eight, sixteen cells, and so on, are formed by continued division, although of course not always quite regularly, until at last a number of cells are formed. This process of the division of the egg-cell is known as the 'segmentation of the yolk,' and is a constant phaenomenon, although it may present various modifications, which are always due to adaptation, and which may be so explained. This is the first course of differentiation in the organism; in place of a single cell, a number of cells, similar to, or different from one another, arise. The functions of the organism, which were all performed previously by the egg-cell, are now performed by the separate cells. The division of the egg-cell must therefore be considered as leading to a division of its functions, although indeed this division is merely a quantitative one. The various stages of this process of division have other relations also, for they appear to agree in character with the mature stage of many lower organisms (Protista), as for example the Volvocinese and Catallacta, in the developmental history of which there is at one time an organism composed of a number of equi-formal cells. The animal organism, therefore, even in the commencement of its ontogeny, passes through several morphological stages, which are permanent among the Protista, and the process of segmentation of the ovum may be explained as a sur- vival transmitted from early ancestors. Accordingly the teleological halo, with which it would
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