IX 



MOLLUSCA 



297 



distinguishable from one another, all four constituting a mass of 

 small cells covering the upper hemisphere of the egg (Fig. 231). 



The ectodermic cap of micromeres extends downwards so as almost 

 to reach the vegetative pole 

 by the process of growth 

 known as epibole (see p. 72), 

 and thus, by epibole, the 

 process of gastrulation is 

 completed. For a time a 

 small opening persists at 

 the vegetative pole, where 

 the rnacronieres are un- 

 covered by micromeres, and 

 this is the blastopore. 



In the meantime the 

 mother cell of the meso- 



derm (4d) has become 

 divided into right and left 

 The proto- 



daughters. 



trochal girdle has now 



ap 



FIG. 231. View of the upper hemisphere of an 

 embryo of Patella coerulea just before hatching. 

 (After Wilson.) 



<i/i, apical cells ; p.tr, prutotrochal cells. 



acquired the form of a 

 complete circle through 

 the acquisition of cilia by 

 those cells which intervene between adjacent groups of primary 

 trochoblasts. These cells may be termed secondary trochoblasts, 

 and two of them in each quadrant are derived from the first 



quartette ; they are in fact 

 daughters of the outer cross 

 cells lq 122 . One secondary 

 trochoblast in each quadrant 

 P is derived from the second 

 ? " quartette; these are the tip 

 cells of the arms of the 

 Molluscan cross, 2q n : and in 

 this way the number of cells 

 entering the prototrochal 

 girdle is raised by three in 

 each quadrant, making a total 

 of 28 (16 + 12). Occasionally 



FIG. 232. Lateral view of a late embryo of Patella the tip Cell divides into two, 



and then there are 32 second- 

 ary trochoblasts. The cilia 

 borne by the secondary trocho- 

 blasts are at first much 

 shorter than those borne by 

 the primary trochoblasts (Fig. 232). By secondary shif tings these cells 

 become rearranged so as to constitute a complete girdle of powerful 

 cilia, behind which is an equally complete girdle of smaller cilia. 



coerulea showing the mode of completion of 

 the prototrochal girdle. (After Wilson, 

 altered.) 



fi.t, primary trochoblasts (darkly shadnd) ; s.t, secondary 

 trochoblasts (dotted). 



