350 PROCEEDINGS OF THE ACADEMY OF [Jul}' 



IV. Conclusions. 



The development of the principal organs and organ systems of 

 Fulgur has now been described, with especial reference to their cell- 

 lineage and topographical relations. It has not been possible to 

 trace the entire cell-lineage of any organ, owing to the very large num- 

 ber of cells which are present before organs appear, but it is possible to 

 determine the quartet and usually the individual cell of the quartet 

 from which any organ arises. In a few cases (cerebral ganglion, velum, 

 intestine) organs may be traced back to individual cells of a stage 

 much later than the quartet formation, but in no case is it possible to 

 observe every division of the cells which enter into the formation of an 

 organ. This lack of a complete knowledge of the cell-lineage is not 

 peculiar to Fulgur, but is general among forms in which the cell-lineage 

 has been studied, and it is not a great hindrance to the study of the 

 localizations of morphogenetic substances and processes of the egg. 

 If groups of cells which give rise to certain organs can be traced back 

 to certain quartet cells, this is usually sufficiently detailed information 

 as to the cellular oiigin of an organ. In Fulgur it is unusually easy to 

 trace this connection between blastomeres and organs, owing to the 

 fact that the organs appear while the blastoderm is still a flat i)late. 



Next to the resemblances between Fulgur and Crepidula in the early 

 cleavages, there is no similarity between these forms more striking than 

 that which is found in the cellular origin of homologous organs. 

 Although the organs of these two genera may differ widely in size and 

 early position, there is not a single instance in which there is any good 

 reason for supposing that these organs have arisen from unlike cleav- 

 age cells of the early stages. On the other hand, there is the best of 

 evidence that homologous organs in Crepidula and Fulgur arise from 

 corresponding cells of the different quartets, and even from correspond- 

 ing cells of much later stages (60-cell stage), even though in still later 

 stages these organs may occupy widely different regions of the embryo, 

 as in the case of the cerebral ganglia. 



AVith regard to the later cleavages, it is not only impossible to follow 

 the lineage of individual cells imtil they give rise to organs, but it is 

 certain that the cells of these two genera cannot be individually com- 

 pared, since there are many more cells in Fulgur at a given stage of 

 differentiation than in Crepidula. For example, there are about 250 

 cells in the embryo of Crepidula at the time of the first appearance of 

 the shell gland, in Fulgur there are about 1,000 cells, and in any com- 

 parison of the cells at this stage it must be remembered that in general 

 four cells of the latter are equal to one of the former. Not onlj^ does 



