374 



INVERTEBEATA 



CHAP. 



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



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



The upper half of 

 the egg continues to 

 divide more rapidly 

 than the lower half. 

 Iqii divides into Iq^", 

 the apical cells, and 

 Iqii'^, which are the 

 so-called "peripheral 

 rosettes" or the An- 

 nelidan cross, whilst 

 the so-called "inter- 

 mediate girdle cells," 

 lqi2, divide into lq^^\ 

 the basal, and Iq^'^^ 

 the intermediate cells 

 of the arms of the 

 " MoUuscan cross." 

 Iq^i and Iq^^ also each 

 divide, so that in each 

 quadrant of the egg 

 there are four daugh- 

 ters of Iq^, and these 



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



trochoblast cells. 



1^1"' 



FiQ. 302. — Later stage in the segmentation of the egg of 

 Phascolosoma qouldii, 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. 



U"' 



Pig. 303. — Two views of tlie 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 " Molhi.scan cross " are 

 ruled with horizontal lines. A, early stage. B, 4S-cell stage, jj./j, 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 



