Decembeb 13, 1907] 



SCIENCE 



827 



the tip of the tail. From this region new ma- 

 terial is continually being produced, out of which 

 the new part is differentiated. The way in which 

 the new part dififerentiates is determined by the 

 pressure relation of the neighboring parts. This 

 pressure relation is the result of the diflferentia- 

 tion with its concomitant pressure relations, that 

 has already taken place in the old part on the 

 one side and of the tension of the new material 

 at the tip on the other side. The new part dififer- 

 entiates therefore into something that is less than 

 the former and more than the latter. In conse- 

 quence there will be an ever-decreasing stimulus 

 and diflferentiation as the new parts are formed, 

 until finally no further stimulus for growth and 

 differentiation is present or is strong enough to 

 act and the growth comes to an end (pp. 280, 

 281). 



To sum up : I have attempted to account for 

 certain phenomena of regeneration by a process 

 of growth in which the following factors appear 

 to enter : ( 1 ) the differentiated material as a 

 factor in limiting the character of new parts; 

 (2) the relation of the cells to each other as a 

 factor in their differentiation, and assume that 

 this relation is due to the mutual pressures or 

 tensions of the cells on each other; (3) the dif- 

 ferentiated cells also determine the existing ten- 

 sion in that part, and this may in turn react on 

 the new cells with which they are in contact. 

 Remove a part and the pressure relations are 

 upset, but this leads ultimately to the reestablish- 

 ment again of the same relations of pressure 

 (p. 282). 



While space does not permit a critical dis- 

 cussion of this hypothesis, one miist admire 

 the author's audacity in putting forward so 

 remarkable a hypothesis without a vestige of 

 evidence to support it. At present his ovm 

 criticism of Geddes and Thomson's theory of 

 sex seems to apply most aptly: 



So vague and general are most of the state- 

 ments . . . that their interpretation belongs to 

 that class of hypothesis, so common in much of 

 our biological speculation, in which the issue is 

 obscured by the appeal to phenomena as uncertain 

 and little understood as the problems that they 

 pretend to explain (pp. 387, 388). 



In the following section, " Experimental 

 Studies in Grafting," the attempt is made 

 to apply this pressure-tension hypothesis to 

 certain phenomena observed in grafts of lower 



animals. The data presented in this section 

 comprise only a small part of those existing. 

 Only the briefest mention is made of the ex- 

 periments on the higher vertebrates. 



" Experimental Studies of the Influence of 

 the Environment on the Life Cycle " form the 

 subject of the fourth section. Here the influ- 

 ence of food on the life cycle in Lepidoptera, 

 the effect of environment on ripening of the 

 sexual organs, alternation of sexual and par- 

 thenogenetic forms in aphids and phylloxer- 

 ans, the influence of environment on the life 

 cycle in the lower Crustacea, in Hydatina 

 and in the Hymenoptera are considered. The 

 section is merely a resume of facts and in- 

 cludes much that is descriptive rather than 

 experimental. 



The faota and theories bearing on the prob- 

 lem of sex determination are presented in the 

 fifth section. Here, too, there is much that 

 is, properly speaking, not experimental, though 

 of value in consideration of the subject in 

 hand. The factors in sex determination are 

 treated under two heads, the external and 

 internal, food being the only external factor 

 discussed. None of the factors discussed 

 prove certainly upon examination to be real 

 factors in sex determination, for the evidence 

 in all cases is either negative, conflicting or 

 of uncertain value. 



In discussion of the relation between the 

 accessory chromosome and sex two possibilities 

 are suggested : the one that the accessory chro- 

 mosome contains the elementary characters, 

 pangenes, determinants, or whatever we may 

 prefer to call them, of the female sex, the 

 other that it produces its results quantita- 

 tively. Morgan points out the difficulties in- 

 volved in the first alternative and maintains 

 that the second affords a much simpler and 

 more plausible basis for interpretation. He 

 suggests that sex may be determined not in 

 the egg or sperm, but "later by the quanti- 

 tative relation resulting from the activity of 

 the chromatin in the cells of the embryo." 

 This hypothesis meets difficulties in those cases 

 where the accessory chromosome has a mate 

 of equal size, for here, as Morgan points out, 

 the quantitative difference does not exist. 



As a matter of fact, there is a third possi- 



