282 THE INVERTEBRATA 



Gastrulation (Fig. 196, 7). The amount of yolk in the macromeres 

 determines the character of the cleavage within certain limits and the 

 type of gastrulation. In forms like Polygordius with very little yolk 

 the micromeres and macromeres are nearly the same size and gastru- 

 lation takes place by invagination; in Aremcola, Nereis and nearly all 

 Polychaeta and Mollusca the micromeres are much smaller than 

 the macromeres, and as they divide to form the ectoderm they grow 

 round the massive macromeres and an **epibolic" gastrula is formed. 

 The cells of the fourth (and fifth) quartettes approach each other 

 from the two sides. The mesoblast cell (d^) begins to withdraw from 

 the surface into the blastocoele, and the blastopore, that is the un- 

 covered surface of the macromeres, becomes much smaller and slit-like. 

 Eventually as gastrulation is completed the lips of the blastopore join 

 in the middle, the same cells meeting each other in every case, leaving 

 an anterior opening which becomes the mouth and a posterior, which 

 closes, but in the neighbourhood of which the anus of the trocho- 

 sphere arises later. The blastopore therefore represents the ventral 

 surface of the larva. At the same time the macromeres withdraw into 

 the interior to form a second cavity, the archenteron, bringing with 

 them the cells of the fourth and fifth quartettes («*, ^^, c^\ a^, c^, d^). 

 The somatohlast (d^) breaks up into a large number of cells to form 

 the ventral plate. 



The change from gastrula to trochosphere (Fig, 197) follows quickly 

 and with little further cell division. The first quartette of micromeres 

 have by this time been diflFerentiated (Fig. 196, 5) into (i) the apical 

 rosette, consisting at first of four small cells and becoming the apical 

 organ of the trochosphere ; (2) the cells of the so-called annelid cross 

 which alternate with those of (i) and form the cerebral ganglia; 

 (3) the prototroch, forming four groups of cells which constitute the 

 preoral ciliated ring of the trochosphere; and (4) the intermediate 

 girdle cells, forming most of the general ectoderm of the part in front 

 of the prototroch, which is called the umbrella. The expansion of the 

 subumbrellar ectoderm, i.e. that behind the prototroch, is due to the 

 proliferation of a single cell in the second quartette of micromeres, 

 d^ (the somatoblast (Fig. 196, 6)). It forms a plate which spreads from 

 its originally dorsal position round the sides, the two wings uniting 

 behind the mouth to form the ventral plate, becoming the ventral 

 body wall. The descendants of this single cell thus make up nearly 

 the whole of the subumbrellar ectoderm. Its sisters a^, b^, c^ give 

 rise to the stomodaeum and are tucked in at the mouth at the close of 

 gastrulation. This marks the completion of the alimentary canal. The 

 young trochosphere now possesses a very thin outer epithelium, 

 thickened in the region of the apical disc and the equatorial ring of 

 cilia, the prototroch, and in the region of the ventral plate, which is 



