CLEAVAGE 227 



egg and the blast omeres formed from it are always divided 

 equally, so that the constant result is a group of similar cells. 

 This is rarely if ever completely realized; the nearest approach 

 to it is seen in the Holothurian, Synapta (Fig. 108). In most 

 examples of so-called equal cleavage slight inequalities may be 

 detected even as early as the two-cell stage (Amphioxus), and 

 quite frequently in the four-, or eight-cell stages. This modifi- 

 cation of equal cleavage is known as adequal. Amphioxus 

 and some of the Echinoderms (Fig. 109) illustrate the fact that 

 no sharp distinction can be drawn between equal and unequal 

 cleavage, for equal cleavage soon becomes unequal, and the 

 transition appears gradually. 



More frequently the cleavage though still total is distinctly 

 unequal, at least by the time eight cells are formed, and often 

 from the very beginning of cleavage. This is characteristic 

 of those telolecithal eggs in which the accumulation of yolk is 

 slightly or moderately marked, as in most of the Platyhelmin- 

 thes, Nemathelminthes, Annulata, Trochelminthes, Mollusca, 

 Ganoids, and Amphibia (Figs. 110, 111). This leads to a second 

 general type of cleavage, the incomplete type, where a portion 

 of the ovum remains uncut by the cleavage planes (Fig. 113). 

 Such eggs are known in general as meroblastic, in distinction 

 from the holoblastic ova whose cleavage is complete. In 

 telolecithal eggs with very large accumulations of deutoplasm, 

 the cleavage planes are nearly restricted to the protoplasmic 

 region and extend only a short distance out into the yolk; 

 this is known as partial cleavage. When the protoplasmic part 

 is quite definitely restricted to the animal pole, cleavage is of 

 an extremely incomplete type known as discoid (Fig. 116), and 

 the result is the formation of a small cap or disc of cells on 

 the surface of the yolk mass (Teleosts, Reptiles, Birds). Or, 

 if the egg is of the centroleciihal type, cleavage is limited to the 

 peripheral protoplasmic layer and is known as superficial 

 (most Arthropods) (Figs. 117, 118). 



Since there are all intermediate conditions between homo-, 

 telo-, and centrolecithal types of ova, we find, as we should 

 expect, all corresponding intermediate conditions between these 



