374 



INVERTEBRATA 



CHAP. 



la 21 



22 



quartette of micromeres ; 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 1 - 1 , 

 the basal, and Iq 1 --, 

 the intermediate cells 

 of the arms of the 



3A 



3 



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

 J'/ifisi-n/iinui/in i/iiitldii, viewed from the posterior aspect. 

 (After Gerould.) 



' Molluscan cross." 



Iq 21 and Iq 2 - also each 

 divide, so that in each 



Cells belonging to the second quartette are dotted; those belonging quadrant 01 the egg 



there are four daugh- 



to the third quartette are ruled with vertical lines. 



ters of Iq' 2 , and these 

 cells are, of course, as in Mollusca and Annelida, the primary 

 trochoblast cells. 



A la 112 



1M 22 



F'IG. 303. Two views of the apical region of the segmenting egg of Phascolosoma 



n/Igarc. (After Gerould.) 



The apical and the prototrochal cells arc loft white. The " peripheral rosettes " or " Annelidan 

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

 ruled with horizontal lines. A, early stage. B, 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 



