CLEAVAGE 229 



poles of the egg and is therefore described as equatorial. In 

 most ova cleaving according to this general rule, the third plane 

 is displaced a variable distance above the equator and is then 

 termed latitudinal. When equatorial this cleavage divides the 

 four equal cells into eight, again equal, arranged in upper and 

 lower groups of four, known as the upper and lower quartets. 

 The fourth cleavage is again meridional and is really double, 

 for two planes appear simultaneously dividing each pair of 

 opposites in each quartet similarly; this results in the formation 

 of upper and lower octets. The fifth cleavage is horizontal or 

 latitudinal, and is again double for it divides simultaneously 

 the upper and lower octets each into two horizontal groups of 

 eight cells, so that the ovum is now r divided into eight vertical 

 rows of four cells each. The cleavages continue to alternate 

 meridionally and vertically, until about the ninth cleavage when 

 512 cells are formed. After this, and in fact usually before this 

 time, the synchronism of cleavage begins to be disturbed, some 

 of the cells dividing more rapidly. 



One of the very frequent causes of departure from this simple 

 schema is the telolecithal character of the ovum. Here the 

 upper quartet is usually smaller than the lower and the fourth 

 cleavage appears earlier in the upper quartet or cells of the 

 animal pole. This leads very soon to an irregularity in the 

 rhythm of cleavage, which may be entirely lost after eight or 

 sixteen cells are formed. 



This typical outline of cleavage serves as a basis to illustrate 

 certain "laws" of cleavage which may be referred to briefly 

 at this point, although their applicability is now known to be 

 very limited. The first of these is the Sachs-Hertwig law 

 describing the geometric relations of the successive cleavage 

 planes. This law really consists of two parts which may be 

 stated as follows: (1) The nucleus of a blast omere (or of any 

 cell) tends to assume a position near the center of the proto- 

 plasmic mass. From this results the equal division of the cell, 

 provided it is free from deutoplasm, or its unequal division 

 if the cell contains deutoplasm not uniformly distributed, for 

 in the latter case the center of the protoplasmic mass does not 



