374 



INVERTEBRATA 



CHAP. 



quartette of rnicromeres ; 3a, 3b, and 3c being formed first and 3d 

 later. They are all comparatively small cells (Fig. 302). 



The upper half of 

 the egg continues to 

 divide more rapidly 

 than the lower half. 

 Iq 11 divides into Iq 111 , 

 the apical cells, and 

 Iq 112 , which are the 

 so-called "peripheral 

 rosettes" or the An- 

 nelidan cross, whilst 

 the so-called " inter- 

 mediate girdle cells," 

 Iq 12 , divide into Iq 121 , 

 the basal, and Iq 122 , 

 the intermediate cells 

 of the arms of the 

 " Molluscan cross." 

 Iq 21 and Iq 22 also each 

 divide, so that in each 

 quadrant of the egg 

 there are four daugh- 

 ters of Iq 2 , and these 

 as in Mollusca and Annelida, the primary 



34 



FIG. 302. Later stage in the segmentation of the egg of 

 Fhascolosoma gouldii, viewed from the posterior aspect. 

 (After Gerould.) 



Cells belonging to the second quartette are dotted ; those belonging 

 to the third quartette are ruled with vertical lines. 



cells are, of course, 

 trochoblast cells. 



A la 1 



112 



la 



FIG. 303. Two views of the apical region of the segmenting egg of Phascolosoma 

 vulgare. (After Gerould.) 



The apical and the prototrochal cells are left white. The " peripheral rosettes "or "Annelidan 

 cross" cells are covered with circles, whilst the " intermediate girdle cells " or " Molluscan cross " are 

 ruled with horizontal lines. A, early stage. 13, 4S-cell stage, p li, polar bodies. 



These cells in Phascolosoma are very large and extend backwards 

 so as to overlap and cover the cells of the second and third quartettes. 

 They become thickly covered with somewhat small cilia. The three 



