362 CELL-DIVISION AND DEVELOPMENT 



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 ^g% may be exactly predicted. Such *' promorphological " rela- 

 tions of the segmenting ^^^ 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 gen- 

 eral rules of cell-division. For this purpose we may consider the 

 cleavage of the ovum under two heads, namely : — - 



1. TJie Geometrical Relations of Cleavage-forms, with reference 

 to the general rules of cell-division. 



2. TJie Promoi'p ho logical Relations of the blastomeres and cleav- 

 age-planes to the parts of the adult body to which they give rise. 



A. Geometrical Relations of Cleavage-forms 



The geometrical relations of the cleavage-planes and the relative 

 size and position of the cells vary endlessly in detail, being modified 

 by innumerable mechanical and other conditions, such as the amount 

 and distribution of the inert yolk or deutoplasm, the shape of the 

 ovum as a whole, and the like. Yet all the forms of cleavage can 

 be referred to a single type which has been moulded this way or that 

 by special conditions, and which is itself an expression of two general 

 rules of cell-division, first formulated by Sachs in the case of plant- 

 cells. These are : — 



1 . The cell typically tends to divide into eqnal pai'ts. 



2. Each new plane of division tends to intersect the pi'cce ding plane 

 at a right angle. 



In the simplest and least modified forms the direction of the 

 cleavage-planes, and hence the general configuration of the cell- 

 system, depends on the general form of the dividing mass ; for, as 

 Sachs has shown, the cleavage-planes tend to be either vertical to the 

 surface {anticlines) or parallel to it {periclines). Ideal schemes of 

 division may thus be constructed for various geometrical figures. In 

 a flat circular disc, for example, the anticlinal planes pass through 

 the radii; the periclines are circles concentric with the periphery. If 

 the disc be elongated to form an ellipse, the periclines also become 

 ellipses, while the anticlines are converted into hyperbolas confocal 

 with the periclines. If it have the form of a parabola, the periclines 

 and anticlines form two systems of confocal parabolas intersecting at 

 right angles. All these schemes are mutatis mutandis, directly con- 

 vertible into the corresponding solid forms in three dimensions. 



