Experimental Studies on Germinal Localization. 19 



the anus does not yet exist. The post-trochal region already shows 

 the mantle fold and the beginning of the shell-gland. On either 

 side the gut may be seen an irregular mass of small cells which 

 I believe to represent the coelomesoblast, though I have not yet 

 traced them to the pole-cells. These masses are not to be con- 

 founded with two masses lying further forward that are pro- 

 liferated off from the ectoblast in two symmetrically placed lateral 

 areas in the pre-trochal region and perhaps represent a part of 

 the paedomesoblast (ectomesoblast) or perhaps the foundations 

 of* the cerebral ganglia. These areas, which are figured by Ko- 

 welevsky ('83, Figs. 32, 37, 55) are shown in the lobeless em- 

 bryos (Figs. 33, 40). 



The ensuing changes take place very much more rapidly In 

 the Naples species (D. entalis) than in the northern form stud- 

 ied by Lacaze Duthiers ('57), which is probably due in a meas- 

 ure to the higher temperature. By the 30th hour the post-trochal 

 region has considerably elongated and the pre-trochal region is 

 somewhat diminished (Fig. 30). In the course of the ensuing 

 twelve hours the pre-trochal region wholly disappears from view, 

 being withdrawn into the interior, while the post-trochal region 

 becomes still more elongated and the larva sinks to the bottom, 

 where it swims only sluggishly. About this time the body becomes 

 surrounded by .an extremely delicate hyaline shell into which 

 the greatly diminished prototroch can be withdrawn; and by 

 the end of the second day the foot appears on the median ventral 

 side. By the end of the third day the foot has become a large 

 protrusible organ, trilobed towards the free end, and the pro- 

 trotroch is still smaller (Fig. 31, which closely agrees with 

 Lacaze's Fig. i, Plate VIII). In many cases the metamorphosis 

 is complete by the end of the fifth day, the prototroch having 

 disappeared, the otocysts and pedal ganglia being clearly visible, 

 and the young Dentaliiim assumes the condition figured by Lacaze 

 on Plate 8, Figs. 2, 3 — a larva of 20-25 days ( !). 



Many details have been omitted from the above account that 

 have already been described In the well-known memoirs of Lacaze 

 Duthiers ('57) and Kowalevsky ('83). Many others will re- 

 quire for their full elucidation much more extended study than 



