576 HAROLD HEATH, 



as a median undivided protuberance. It probably is formed to a 

 slight extent by the second and third quartettes of the anterior side 

 of the embryo, and to a greater degree by these same quartettes of 

 the posterior side which have grown round to form the ventral sur- 

 face. On its anterior edge arises the opening of the "foot gland" 

 (KowALEvsKi), an organ probably similar, in function at least, to its 

 namesake in other Molluscs. 



About the time of the formation of the foot the first indication 

 of a shell appears on the dorsal surface, consisting of rows of cells 

 which are destined to secrete the calcareous salts alternating with 

 large cells apparently containing some mucous-like secretion. A 

 cuticle forms above this region in whicli the valves of the shell and 

 the spicules of the mantle are deposited. This shell is a cuticular 

 structure constituting the tegmentum, the articulamentum arising much 

 later. At first it occupies a position posterior to the prototroch but 

 gradually it extends on to the head vesicle and finally its anterior 

 borders occupy a position but little posterior to the apical sense 

 organ. 



The larva which escapes from the membranes before the cal- 

 careous portion of the shell commences to appear has much the 

 appearance of the adult, the hemispherical form of the head vesicle 

 constituting the most apparent difference. This gradually flattens 

 however and ultimately forms the proboscis and the anterior part of 

 the mantle furrow in the manner indicated by the figures. This 

 change completes the essential features of the metamorphosis. 



The parts the various quartettes play in the developmental pro- 

 cesses are given in the inserted table. 



5. Nomenclature. 



Throughout I have used Conklin's modification of Wilson's 

 system of nomenclature in which the first quartette of micromeres 

 and all their derivatives are designated by the coefficient i {la, id, 

 lai-^, Ic'^'^ etc.), the second quartette and its progeny by the 

 coefficient 2 {2a, '2d, 2 c'^- 1 etc.) the third quartette by the coefficient 

 S {3a, 3d etc.), and the fourth quartette by 4 {4a, 4d etc.). 



"The animal and vegetal poles are considered the fixed points 

 in the egg. In the ectoblast the stem or parent cell is in all cases 

 the upper one. The stem cell in the entoblast and mesoblast is in 

 every case the lower one. If, in any case, the cleavage is meridional 

 (an exceedingly rare thing) the right moiety is considered the stem 



