296 



INVEETEBEATA 



CHAP. 



The division of the apical cells Iq 111 results in the formation of a well- 

 marked plate of eight central cells at the animal pole (Fig. 231). 



B 



D 



3D 



FIG. 229. Two stages in the gastrulation of Patella coerulea in optical longitudinal section. 

 The residual macromeres are shaded. (After Wilson. ) 



A, stage showing the formation of the mesoderm mother cell, 4(7, and the inward migration of the 

 macromere, 3D. B, stage showing the division of the left nu-soderm mother cell, and the inward 

 migration of the other macromeres. ap, apical plate ; ji.tr, prototroch. 



Meanwhile other 

 changes take place. The 

 four groups of primary 

 trochoblasts develop 

 transverse rows of power- 

 ful cilia, each row bearing 

 a striking resemblance 

 to a single "comb" of 

 a Ctenophore, and the 

 apical cells develop a 

 terminal tuft of long 

 stiff cilia. The embryo 

 at this stage, when looked 

 at from above (Fig. 230), 

 bears a striking resem- 

 blance to a larval Cteno- 

 phore ; and this stage, 

 which is reached eight 

 hours after fertilization, 

 is termed by Wilson the 

 Ctenophore stage. 



The cells constituting the apical plate do not appear to divide 

 further, but the cells forming the arms of the Molluscan cross under- 

 go repeated divisions, in consequence of which these arms cease to be 



1221 



FIG. 230. The " Ctenophore " stage in the development 

 of J'nti-Uii cni'i-iileii. Seen from above. The cells 

 are marked as in Fig. 227. (After Wilson, somewhat 

 altered.) 



