232 GENERAL EMBRYOLOGY 



as to reduce their exposed surfaces to a minimum. But there 

 are frequent exceptions here as in the case of the other "laws" 

 of cleavage. Furthermore, the active migration of blastomeres 

 is not infrequent, so that cells may ultimately be found in 

 regions considerably removed from the place of their formation 

 (Rotifers, Molluscs). (3) Rhythm. The rate of division fre- 

 quently does not correspond with the relative amount of 

 deutoplasm. The factors regulating the rhythm of division 

 still remain largely unknown. It is true here as in many other 

 "exceptional" cleavage phenomena, that the deviation is 

 related to the future morphological or functional character 

 of the developing organism or of parts of it. We shall return 

 to this aspect of cleavage later. 



With these general considerations in mind we may proceed 

 now to a more exact description and classification of the geo- 

 metric forms of cleavage. Here we shall find illustrations of 

 many of the preceding statements. Considering first the various 

 forms of complete cleavage (holoblastic ova), we may distin- 

 guish rather roughly, four types, radial, spiral, bilateral, 

 irregular. 



(1) Radial. This is the form exemplified by Synapta (Fig. 

 108), already described as being geometrically the simplest. 

 This should perhaps better be termed rotatorial than radial, for 

 while the blastomeres are arranged in symmetrical fashion in 

 any single plane perpendicular to the main axis of the egg, 

 there are usually considerable differences in size between the 

 cells of the animal and vegetal poles. Cleavage of this type 

 is found in the sponges, jelly-fishes, and many Echinoderms, 

 in some Nematodes and Rotifers. In the sea-urchins (Fig. 109) 

 the third cleavage is meridional in the upper quartet, in the 

 lower latitudinal and very unequal, cutting off a quartet of very 

 small cells or micromeres which curiously are found at the lower 

 or vegetative pole. 



(2) Spiral. This may be regarded as a modification of the 

 radial type resulting from the displacement of cells so that the 

 blastomeres above and below any horizontal cleavage furrow 

 tend to alternate with one another in a vertical direction, some- 



