248 GENERAL EMBRYOLOGY 



tween blastomeres and differentiated groups of cells in the later 

 embryo, we may describe briefly a typical instance, using as the 

 subject one of the simpler and more regularly cleaving types, 

 the Turbellarian, Planocera, as described by Surface. This 

 account of the cleavage of this form should be read with the 

 expectation of finding frequent " exceptions" to the laws of 

 cleavage mentioned above. 



In Planocera (Figs. 119, 120) cleavage is total, unequal, and 

 spiral (dexiotropic). The first plane is meridional and divides 

 the egg into two adequal blastomeres known as AB and CD. 

 The division of each of these is also meridional but is unequal, 

 each forming a smaller and a larger cell, and dividing the entire 

 ovum into two larger, and slightly unequal, cells known as B 

 and D, and two smaller cells, also slightly unequal, known as 

 A and C. Of these D is the largest, and from later develop- 

 ment is known to be posterior in position; B is anterior, A on the 

 left, and C on the right, as viewed from the animal pole and 

 with reference to later structure. 



The third cleavage is horizontal, unequal, and strongly 

 spiral (dexiotropic). As usual among the Turbellaria the 

 larger cells divide shortly before the smaller. On account of the 

 size differences between the cells of the upper and lower 

 quartets, we may describe the upper quartet of smaller cells 

 (micromeres) as budded off from the lower quartet of macro- 

 meres. The quartets of micromeres are designated by small 

 letters, the macromeres by capitals. Thus in the eight-cell 

 stage we have the first quartet of micromeres, la, Ib, Ic, Id, 

 and the first quartet of macromeres, 1A, IB, 1C, ID. Successive 

 quartets are designated by numerical coefficients, products of 

 the division of the quartet cells by exponents. Thus in passing 

 from eight cells to sixteen the macromeres bud off, this time 

 in a Iseotropic direction, a second quartet of micromeres, 2a, 2b, 

 2c, 2d, the macromeres themselves remaining known now as 2 A, 

 2B, 2C, 2D. Shortly thereafter the first quartet of micromeres 

 divide, also laeotropically, forming two groups of four cells each 

 known as la 1 , Ib 1 , Ic 1 , Id 1 , and la 2 , Ib 2 , Ic 2 , Id 2 ; the cells lying 

 toward the animal pole are designated by the lower exponent. 



