ONTOGENESIS 159 



period of development, as, for instance, a bird. 

 In the latter case, the amount of reserve food is 

 so great that it bulks far more than the proto- 

 plasm itself, and crowds the greater part of the liv- 

 ing substance to one side of the cell. On the other 

 hand some eggs contain so little food yolk that they 

 are almost transparent, and what metaplasm there 

 is is evenly distributed throughout the egg-cell. 

 Such an egg is called isolecithal. In the eggs of 

 many animals, even before cleavage begins, the pro- 

 toplasm may be seen to be visibly differentiated in 

 different regions which are destined to develop into 

 various parts of the organism. There appears to 

 be every gradation in the degree of such differen- 

 tiation. In some cases there is apparently none at 

 all. In others, different zones or strata may be 

 seen. In many such eggs mutilation of certain parts 

 of the egg-cell produces corresponding defects in the 

 resultant organism, showing that the protoplasm was 

 already specifically differentiated before cleavage. 



The amount and distribution of the food-yolk is 

 of the greatest influence in determining the nature 

 of the cleavage process. In an isolecithal egg the 

 first cleavage usually divides the egg-cell into two 

 approximately equal daughter-cells, which, in turn, 

 divide, each into two others of about the same size. 

 This process is repeated in rapid succession until 

 the original zygote has been halved, quartered, and 

 then cut into 8, 16, 32, 64, 128 cells, etc., the process 

 continuing until the first step in differentiation 

 occurs. The result of such repeated cell division 



