CLEAVAGE 249 



The fifth cleavage, dividing the sixteen cells into thirty-two, 

 in general resembles the preceding but is dexiotropic through- 

 out. As the result w have the macromeres, 3 A, 3B, 3C, 3D; 

 a third quartet of micromeres, 3a, 3b, 3c, 3d; the second quartet 

 of micromeres divides into 2a*, 2b\ 2c*, 2d*, and 2a 2 , 2b 2 , 2c 2 , 

 2d 2 , while the eight cells previously derived from the first 

 quartet of micromeres now form la 11 , la 12 , la 21 , la 22 , Ib 11 , lb 12 , 

 lb 21 , lb 22 , lc 11 , lc 12 , lc 21 , lc 22 , Id 11 , Id 12 , Id 21 , Id 22 . 



After thirty-two cells have been formed in this fairly regular 

 fashion, the rhythm of cleavage becomes modified so that there 

 follow stages of 40, 44, 45, 53, 61 cells, etc. 



It is unnecessary for us to go farther with the details of 

 these cleavages save in one particular. After the thirty-two- 

 cell stage the macromeres divide again unequally giving off a 

 group of large cells which contain most of the deutoplasm of 

 the original ovum. In spite of their size relations these large 

 cells are known as the fourth quartet of micromeres, 4a, 4b, 4c, 

 4d, and the remaining smaller cells as the fourth quartet of 

 macromeres, 4A, 4B, 4C, 4D. This contradiction in termin- 

 ology is justified by the later history of these cells. 



We may now consider the fates of these various groups of 

 blastomeres. Quoting from Surface, "From the first quartet 

 [of micromeres] arises the ectoderm, covering the anterior 

 and dorsal portions of the body. From cells of this quartet four 

 strings of cells bud into the interior of the embryo and form the 

 ganglion. The eyes arise in ectodermal cells of this quartet. 

 The second quartet gives rise to the larger portion of the ecto- 

 derm on the ventral and posterior regions of the body. From 

 cells of this quartet is formed most of the ectodermal pharynx. 

 A portion of the second quartet is budded into the embryo and 

 forms mesoderm. From this source arises probably only that 

 mesoderm formed around the blastopore and which is later 

 concerned in the structures of the pharynx. 



"The third quartet consists of small cells from which appar- 

 ently only ectoderm is derived. The individual divisions of 

 these cells have not been traced very far, but there is every 

 reason to believe that they form ectoderm only. 



