614 HAROLD HEATH, 



which brings the area of the most active multiplication of cells im- 

 mediately posterior to the shell, a position which it occupies iu 

 Gastropods, and its situation corresponds closely to the same region 

 in Annelids. 



The increase in area of these posterior third quartette tracts is 

 not, except to a slight extent, in arcs of circles parallel to the proto- 

 troch, though there is some lateral displacement caused by the 

 growth of the first somatoblast on the dorsal side. The main growth 

 is at right angles to the velum, and the distance on the dorsal side, 

 between the posterior border of the blastopore and the prototroch, 

 becomes continually increased. Also through invagination the distance 

 between the blastopore and velum on the ventral side becomes les- 

 sened, and in proportion as the former shifts toward its future position 

 these posterior third quartette areas become extended beyond the 

 original position of the blastopore up along the ventral side until they 

 come in contact with the prototroch, thus forming most of the ventral 

 side of the future embryo (diagram E, d, f, page 644). 



Probably about half of the trunk of the embryo is formed of 

 these posterior third quartette cells. Each group contributes to the 

 lateral borders of the dorsal surface, the products of the first somato- 

 blast forming the mid-dorsal region : the posterior lateral regions arise 

 wholly from these cells, as does also most of the ventral surface. 

 Diagram E will show more clearly the position of this quartette. 



As regards the third quartette cells of the anterior quadrants, 

 little remains to be said. In their first cleavages a very marked 

 regularity exists but this gradually disappears and an irregular group 

 of cells results. Cell divisions continue but the increase of superficial 

 extent in the groups is very small, apparently little more than in the 

 earlier stages, I have studied the matter quite closely and am con- 

 vinced that they always remain thus relatively small and are of 

 comparatively little importance in the history of the embryo. They 

 have been traced up to a point where the blastopore has almost 

 reached its permanent position and at such a time they form two 

 areas on each side of the mouth in juxtaposition with the prototroch 

 as shown in diagram E, c. It is possible that they include the 

 antero-lateral borders of the foot when it is formed. In any event 

 they probably function as simple ectodermal cells many if not all of 

 which lie between the foot and shell, and consequently in the meta- 

 morphosis they must in great part lie in the mantle furrow. 



