14 OUTLINES OF CHORDATE DEVELOPMENT 



This typical arrangement of cleavage planes is by no 

 means invariable. Variations are indeed very frequent but 

 the cells are usually arranged according to definite plans. In 

 many cases the cleavage pattern is nearly radial, and in others 

 the blastomeres may shift more or less giving the appearance 

 of a spiral cleavage. In some eggs the micromeres of the 

 sixteen-cell stage may not all divide similarly but four may 

 divide vertically and four horizontally. These variations in 

 cleavage do not affect subsequent processes. 



As a consequence of the early blastomeres remaining well 

 rounded a central space is formed among them. Virtually 

 present in the four-cell stage, this space becomes real at eight 

 cells, and as the cells multiply and gradually lose their rounded 

 form they push away from the center of the mass leaving a 

 definite space within, at first open at the poles (cleavage pores) . 

 This is the beginning of the segmentation cavity or blastoccel. 

 By the time thirty-two cells are formed the poles close over 

 and soon the blastoccel is entirely closed. From about sixty- 

 four cells on (Fig. 5, E, F) the blastomeres lose their rounded 

 outline and become flattened and closely packed in a simple 

 epithelial layer. This arrangement of the blastomeres may be 

 taken as the beginning of the blastula stage, which may be 

 considered fully established when the number of cells reaches 

 one hundred and twenty-eight. The hollow spherical blastula 

 of this type (Fig. 5, F) is called a cceloblastula, and it is com- 

 monly regarded as the most primitive type of blastula. In 

 Amphioxus this is bilaterally symmetrical and the epithelial 

 wall is of varying thickness on account of the varying sizes of 

 the cells. The cells are relatively free from deutoplasm, and 

 therefore smaller, in the anterior region, richer in deutoplasm, 

 and therefore larger, in the posterior region, and uniformly 

 graded in size and deutoplasmic content between these regions. 

 The animal pole remains antero- ventral (Fig. 7, J5). The 

 blastocoel is very large and, on account of the varying thickness 

 of its wall, may be said to be slightly eccentric toward the 

 anterior side. 



