GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS 265 



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 w^hich the embryo will finally appear in 

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

 tions 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 gen- 

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

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



1. TJic Geometrical Relations of Cleavage-fonns, with reference 

 to the general laws of cell-division. 



2. T/ie PromorpJiological 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 are 

 variants of a single type which has been moulded this way or that 

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

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

 cells. These are : 



1. TJie cell typically tends to divide into equal parts. 



2. EacJi new plane of division tends to intersect the preceding plane 

 at a rig J it 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 



