364 CELL-DIVISION AND DEVELOPMENT 



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

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

 be related to the typical division of a sphere in accordance with Sachs's 

 rules. 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 ^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 clearest 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 ^gg of the holothurian Synapta, as shown in 

 the diagrams, Fig. 169, constructed from Selenka's drawings. The 

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

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

 meridional, cuts the first plane at right angles and divides the ^gg 

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

 ^gg into equal octants. The order of division is thus far exactly 

 that demanded by Sachs's rule 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 walls consisting of a single layer of cells in 

 which anticHnal 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. 



Hertwig s Development of Sachs's Rules. — Beside Sachs's rules 

 may be placed two others formulated by Oscar Hertwig in 1884, 

 which bear directly on the facts just outHned and which lie behind 

 Sachs's principle of the rectangular intersection of successive division- 

 planes. These are : — 



1 . The nucleus tends to take np a position at the centre of its sphere 

 of influence, i.e. of the protoplasmic mass in zvhich it lies. 



2. The axis of the 7nitotic figtires typically lies in the longest axis 

 of the protoplasmic mass, and division therefore tends to cut this axis 

 at a right a^tgle. 



The second rule explains the normal succession of the division- 



