viz ANNELIDA 133 



verted into a gastrula, and then into a Trochophore, by rearrange- 

 ments of cells, without further cell-division. Further divisions of 

 cells only occur after the Trochophore has been feeding for some 

 time. 



Up to the 64-cell stage then, all the cells of the egg divide 

 simultaneously, so that the first micromeres divide once as the second 

 quartette of micromeres is given off. When the division to form the 

 third quartette of micromeres is complete, the first quartette have 

 divided twice; and when the fourth quartette, which gives rise to 

 mesoderm and endoderm, is formed, they have divided thrice, and with 

 this division the 64-cell stage is attained. 



The egg, which from the 16 -cell stage had taken on the form of 

 a hollow spherical blastula, now begins to flatten out. In the same 

 period the second quartette of micromeres has divided twice and the 

 third quartette once. 



At the next cleavage the cells of the prototroch fail to divide 

 and so do certain other cells, descendants of the first group of 

 micromeres, but all the other cells of the egg divide. A fifth 

 quartette is given off, which is destined to form endoderm only, and 

 the residual macromeres are now barely if at all larger than the 

 micromeres to which they gave rise. The flattening of the embryo 

 continues till it assumes the form of a flattened plate (Fig. 101, D). 



The next cleavage is participated in only by the macromeres, 



. which divide not spirally but symmetrically with regard to the future 



median plane of the embryo. The fifth quartette of micromeres also 



divide, so do the fourth, and some cells of the second and third ; and 



then invagination commences. 



-In describing in detail the divisions of the cells it is most 

 convenient to deal with the different quartettes of micromeres 

 separately. Since, as has been already stated, it is impossible in the 

 earlier divisions to distinguish one macromere from another, it is 

 convenient to be able to refer to them collectively, and the letter q is 

 used to denote a, b, c, and d. 



In all four quadrants of the egg the divisions of the first 

 quartette of micromeres are exactly alike. We say that Iq divides 

 into upper cells Iq 1 , mothers of the whole upper hemisphere of 

 the Trochophore above the prototroch, and into lower cells Iq 2 , 

 the mothers of the prototroch ; Iq 1 divides again into upper cells 

 Iq 11 , mothers of the apical plate, or as it is sometimes termed the 

 rosette, and of a St. Andrew's cross of cells radiating from it, called 

 the " Annelidan cross " because it is conspicuous in the eggs of all 

 Annelids ; and into lower cells Iq 12 , mothers of a group of cells called 

 by Wilson the intermediate girdle cells but termed by other authors 

 the Molluscan cross, because in Molluscan eggs these cells take on 

 the form of a conspicuous upright cross. Iq 2 divides into Iq 21 , and 

 Iq 22 , one set of cells lying obliquely above the other. 



At the next cleavage Iq 11 divides into Iq 111 , forming the apical 

 cells or rosette which carry the tuft of long stiff cilia which con- 



