GENERAL CONSIDERATIONS 283 



important physiological events accompany the earlier cleavage stages of de- 

 velopment (Nelsen, '48, '49). 



Aside from the foregoing examples which demonstrate that invisible changes 

 in the developing blastula are associated with morphological transformations 

 is the fact that experimental research has demonstrated conclusively that an 

 organization center is present in the very late blastula and beginning gastrula. 

 The organization center will be discussed later. However, at this point it is 

 advisable to state that the organization center is the instigator and the con- 

 troller of the gastrulative processes, and gastrulation does not proceed unless 

 it is developed. 



The above considerations suggest that the period of cleavage and blastu- 

 lation is a period of preparation for the all-important period of gastrulation. 

 Other characteristics of this phase of development will be mentioned in the 

 chapter which follows. 



4. Geometrical Relations of Early Cleavage 

 a. Meridional Plane 



The meridional plane of cleavage is a furrow which tends to pass in a 

 direction which, if carried to completion, would bisect both poles of the egg 

 passing through the egg's center or median axis. The latter axis theoretically 

 passes from the midpolar region of the animal pole to the midpolar region 

 of the vegetal pole. The beginning of the cleavage furrow which follows the 

 meridional plane may not always begin at the animal pole (fig. 1400) al- 

 though in most cases it does (figs. 142A-C; 154A-C; 155A). 



b. Vertical Plane 



A vertical plane of cleavage is a furrow which tends to pass in a direction 

 from the animal pole toward the vegetal pole. It is somewhat similar to a 

 meridional furrow. However, it does not pass through the median axis of the 

 egg, but courses to one side of this axis. For example, the third cleavage planes 

 in the chick are furrows which course downward in a vertical plane; paral- 

 leling one of the first two meridional furrows (fig. 155C). (See also figs. 153D; 

 154E relative to the third cleavage furrows of the bony ganoid fishes, Amia 

 calva and Lepisosteus (Lepidosteus) osseus.) 



c. Equatorial Plane 



The equatorial plane of cleavage bisects the egg at right angles to the 

 median axis and halfway between the animal and vegetal poles. It is never 

 ideally realized in the phylum Chordata, and the nearest approach to it is 

 found, possibly, in one of the fifth cleavage planes of the egg of Amby stoma 

 maculatum (fig. 149F) and the first cleavage plane of the egg of the higher 

 mammals (fig. 145A). 



