GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS 26y 



the same principle to the cleavage of animal cells. The discoid or 

 spheroid form is more or less nearly realized in the thalloid growths 

 of various lower plants, in the embryos of flowering plants, and 

 elsewhere (Fig. 119). The paraboloid form is according to Sachs 

 characteristic of the growing points of many higher plants; and 

 here too the actual form is remarkably similar to the ideal scheme 

 (Fig. 119, /). 



For our purpose the most important form is the sphere, which is 

 the typical shape of the egg-cell; and all forms of cleavage are deriv- 

 atives of the typical division of a sphere in accordance with Sachs's 

 laws. The ideal form of cleavage would here be a succession of 

 rectangular cleavages in the three dimensions of space, the anticlines 

 passing through the centre so as to split the i^gg in the initial stages 

 successively into halves, quadrants, and octants, the periclines being 

 parallel to the surface so as to separate the inner ends of these cells 

 from the outer. No case is known in which this order is accurately 

 followed throughout, and the periclinal cleavages are of compara- 

 tively rare occurrence, being found as a regular feature of the early 

 cleavage only in those cases where the primary germ-layers are sepa- 

 rated by delamination. The simplest and most typical form of egg- 

 cleavage occurs in eggs like those of echinoderms, which are of 

 spherical form, and in which the deutoplasm is small in amount and 

 equally distributed through its substance. Such a cleavage is beauti- 

 fully displayed in the Qgg of the holothurian Synapta, as shown in 

 the diagrams. Fig. 120, constructed from Selenka's drawings. ^ The 

 first cleavage is vertical, or Jiuridional, passing through the egg-axis 

 and dividing the egg into equal halves. The second, which is also 

 meridional, cuts the first plane at right angles and divides the egg 

 into quadrants. The third is horizontal, or equatorial, dividing the 

 (tgg into equal octants. The order of division is thus far exactly 

 that demanded by Sachs's law and agrees precisely with the cleavage 

 of various kinds of spherical plant-cells. The later cleavages depart 

 from the ideal type in the absence of periclinal divisions, the embryo 

 becoming hollow, and its wall consisting of a single layer of cells in 

 which anticlinal cleavages occur in regular rectangular succession. 

 The fourth cleavage is again meridional, giving two tiers of eight 

 cells each ; the fifth is horizontal, dividing each tier into an upper 

 and a lower layer. The regular alternation is continued up to the 

 ninth division (giving 512 cells), when the divisions pause while the 

 gastrulation begins. In later stages the regularity is lost. 



This simple and regular mode of division forms a type to which 

 nearly all forms of cleavage may be referred ; but the order and form 



1 Cf. also Fig. 3. 



