748 



SCIENCE. 



[N. S. Vol. XX. No. 518. 



2. When an alternative exists at the cut 

 end we meet with the problem of polarity 

 also. In the hydroid, Tubularia, an anal- 

 ysis of the conditions leads me to conclude : 

 (1) That the hypothesis of stuffs moving 

 in given directions does not explain the 

 facts, (2) that the results can be accounted 

 for on the ground of the amount of nutri- 

 tive substance present in the pieces, taken 

 in connection with the relative conditions 

 of the stem at each level. Furthermore, in 

 this case the stimulus of the water on the 

 exposed end, calling forth hydranth regen- 

 eration, is an important factor in the result. 



3. There is nothing in the phenomena to 

 suggest that the old part has a stereometric 

 influence, i. e., a directive influence on the 

 new part, as the term 'polarity' suggests. 

 On the contrary, the influence is largely 

 centripetal in direction, so far as there is 

 any question of direction involved. 



4. An analysis of the conditions present 

 in lateral regeneration in planarians sug- 

 gests that at least three separate factors 

 are to be recognized in the changes that 

 take place. I have put these factors into 

 the categories of (a) totipotence, (&) 

 heterotropy and (c) organization. 



5. The ends of the old organs have also 

 an influence on the regeneration, but a less 

 important one in some cases than those 

 just mentioned. 



6. The same factors are also present in 

 antero-posterior regeneration in which an 

 alternative is present. When «o alterna- 

 tive exists the totipotence has certain 

 limitations, which depends, however, on 

 the special combination of tissues in the 

 new part, rather than on any limitations 

 in each group of cells. 



7. The organizing principle acts on the 

 new and old part as a whole and determines 

 the relative arrangement and proportions 

 of the new organs. 



T. H. Morgan. 

 Columbia Unweesitt. 



MOSAIC DEVELOPMENT IN TEE ANNELID 

 EGG-.* 



OuE general interpretation of the prob- 

 lem of development has been somewhat 

 prejudiced by the fact that so much of the 

 earlier experimental work dealt with such 

 eggs as those of echinoderms, medusje, 

 AmpMoxus or the nemertines, where any 

 one of the first two or four cells may pro- 

 duce a perfect dwarf embryo; for such 

 cases seem at first sight to be irreconcilably 

 opposed to any theory of definite pre- 

 localization or mosaic development. The 

 collapse of the Roux-Weismann theory of 

 differentiation by qualitative nuclear divi- 

 sion discredited for a time the whole 

 mosaic theory; but more recent experi- 

 mental work, especially on the eggs of 

 ctenophores and mollusks, promises to re- 

 establish it on a new basis. In the course 

 of the past year I have been able to show 

 experimentally that the development of 

 mollusks {Dentalium, Patella) conforms in 

 its main features to the mosaic principle, 

 and, furthermore, that the cleavage mosaic 

 is foreshadoAved by a very definite original 

 pre-loealization of specific protoplasmic 

 materials in the undivided egg. During 

 the past summer I have had an opportunity 

 to extend these observations in some meas- 

 ure to the egg of an annelid, where the 

 same general principle has been found to 

 hold true. 



The development of the annelids presents 

 the problem in a very clear-cut form, since 

 from the first cleavage onward the prin- 

 cipal material of the segmented trunk- 

 region lies in the posterior cell of the em- 

 bryo, and this cell is in most species some- , 

 what larger than the anterior, and hence 

 may be immediately identified. The ex- 

 periments here reported consist in a com- 

 parison of the development of the isolated 

 posterior cell of the two-cell stage with 



* Read before the National Academy of Sci- 

 ences, November 16, 1904. 



