1 1 4 CONK LIN. [Vol. X 1 1 1 . 



of the cross as such landmarks ; and as long as any portion of 

 them can be seen the cells of these quartettes can be identified, 

 but when the ectoblast has grown around the ^^^ so that the 

 arms of the cross are no longer visible from the ventral side, I 

 have found it impossible to identify individual cells. Conse- 

 quently I have not traced many of the cells of this quartette 

 directly to the organs which they form, though I have followed 

 the lineage until there are eleven cells of the second quartette 

 in each quadrant and six of the third, or sixty-eight cells in all. 

 A few cells of the second quartette could be traced farther than 

 this, owing to their relation to the anterior arm of the cross. 



The derivatives of these two quartettes form all of the ecto- 

 dermal covering of the body posterior to the prototroch, and in 

 addition they give rise to the ectodermal portions of the follow- 

 ing specific structures : a large part of the velum ; the blasto- 

 pore, stomodaeum, and mouth ; a region of apical growth at the 

 posterior end of the embryo, the anal cells and proctodaeum, 

 the external excretory cells, the shell gland, foot, and otocysts, 

 the branchial chamber, gills, and larval heart. It will thus be 

 seen that a large part of the important organs, both of the larva 

 and of the adult, are derived from these two quartettes. 



The prototroch forms a convenient and fairly accurate boun- 

 dary between the cells of the first quartette on the one side 

 and those of the second and third quartettes on the other. A 

 glance at Figs. yZ and 80 will show that the portion of the 

 larva posterior to the velum is much larger than that anterior 

 to it, and at the time when the larva changes into the adult the 

 portion of the body anterior to the velum becomes very small 

 and almost disappears, while the region posterior to the velum 

 gives rise to practically the entire body. 



I. The Second Quartette. 



In the formation of the adult body this group of cells is per- 

 haps the most important of any in the entire &g^. Knowing 

 this fact, I have done my best to trace the lineage of these 

 cells as far as possible, but in spite of prolonged effort I have 

 not been able to carry the lineage beyond a stage in which 



