42 THE AMERICAN MUSEUM JOURNAL 



The fourth cleavage (Model lo) is also oblique. It results in 

 the separation of another quartet of small protoplasmic cells 

 slightly to the left of the large yolk-laden cells and also at the 

 animal pole (Model 1 1 ) . 



The fifth cleavage is simply a division of the first quartet of 

 small cells (Model 12). 



By the sixth cleavage, the beginning of which is shown in 

 Model 12, a third and last quartet of similar small cells is given 

 off at the animal pole. This cleavage also is oblique, but to the 

 right. By this alternation in the direction of each cleavage 



MODEL 11 



Fourth cleavage completed. A second quartet of ectoblasts formed. 

 Division beginning in cells of first quartet. Fifth cleavage begun. 



plane, which began with first cleavage as indicated by the 

 rotation of nuclei to the right, or in a clockwise direction on 

 Model 4, the symmetrical arrangement of the cells is maintained. 

 Lying at the animal pole of the egg, these three quartets of small 

 cells form the so-called dorsal plate, which, by rapid multiplica- 

 tion of cells by division, is destined to grow until it completely 

 covers the egg and forms the outer layer or ectoderm of the 

 embryo. These cells are therefore known as "ectoblasts." 



[18] 



