CLEAVAGE. 49 



for different individuals. In other species this variability begins much earlier 

 in the segmentation series. 



In addition to variations in the number and size of the cells above de- 

 scribed, variations also occur in the direction of the cleavage planes and in the 

 relations of the resulting cells. In all eggs except centrolecithal, the first two 

 cleavage planes bisect each other at right angles through the protoplasmic pole. 

 Subsequent cleavage may follow one of three types, which are distinguished as 

 radial, spiral and bilateral. 



Radial cleavage is the simple type of cleavage already described as occurring 

 in Synapta and in the frog (Figs. 25 and 27). 



Spiral or oblique cleavage occurs chiefly in Worms and Mollusks. In this 

 form of cleavage the first and second cleavage planes bear the same relation to 

 the egg and to each other as in radial cleavage. The first horizontal cleavage 

 is not, however, a continuous horizontal plane but cuts the cells in such a manner 

 that the four lower cells have a position as if rotated one -half cell, usually to the 

 left. The divisions between the cells of the lower row thus alternate with those 

 of the upper row, like layers of bricks in a wall. The next horizontal cleavage 

 plane is also oblique but is at right angles to the preceding and results also in 

 alternation of all the cells. This regular alternation of spiral cleavage planes 

 may continue for some time, but, as a rule, the cleavage soon becomes very 

 irregular and there is usually much variation in size of the blastomeres. 



In the bilateral form of cleavage seen in Tunicates and Cephalopods, after 

 the first cleavage, the cells segment symmetrically on each side of the first 

 plane. 



In some forms the cleavage appears to follow definite rules as regards the 

 number of cells which result from each segmentation. This is known as deter- 

 minate cleavage. In other cases, after a number of divisions, the number of 

 cells resulting from each cleavage is indefinite, that is, it varies for each indi- 

 vidual. This is designated indeterminate cleavage. 



Reviewing the results of cleavage, it is to be noted that in every case there is 

 formed a larger or a smaller group of cells. In the case of equal holoblastic 

 cleavage, these cells are all of the same or of nearly the same size, and constitute 

 what is known as the morula or mulberry mass (Fig. 25, E). A similar condition 

 obtains in unequal holoblastic cleavage with the one exception, that there is 

 a marked difference in the size of the cells constituting the morula (Fig. 27). 

 In superficial meroblastic cleavage the group of cells forms a layer enclosing the 

 central yolk, the latter being unsegmented but containing some nuclei. In 

 discoidal meroblastic cleavage the group of cells spreads itself over a limited 

 superficial area, while beneath it lies the large mass of unsegmented yolk, con- 

 taining, however, some nuclei (Figs. 28 and 29). 



After the morula has become-fully formed, there appears in it a cleft or cavity 



