Febeuaey 25, 1898.] 



SCIENCE. 



285 



•development in his earlier papers, and of the 

 grounds for his later conclusions in favor of the 

 qualitative nature of cleavage and the ' self- 

 differentiation ' of the blastomeres. This re- 

 view is most satisfactory in its spirit of fairness 

 and in its appreciation of the magnificent work of 

 Eoux, and stands in refreshing contrast in these 

 respects to much recent scientific (?) discussion of 

 this investigator's views on the other side of the 

 Atlantic. The author then proceeds to develop 

 the difficulties in Eoux's theory and presents 

 grounds for a different view. He points out that 

 in all cases, except the ctenophore egg (and the 

 unmentioned gasteropod egg), it has been shown 

 that the early blastomeres have each the power 

 to produce the whole embryo, though under 

 certain circumstances they may not do so. The 

 author believes that there is no profound differ- 

 ence in principle between the conditions in the 

 ■ctenophore (and gasteropod?) egg and elsewhere; 

 the divergent results in this case, he thinks, 

 may be explained by the fixity of the proto- 

 plasmic forms in the ctenophore egg, or some 

 kindred condition. This totipotence of the em- 

 bryonic cells may persist, Professor Morgan be- 

 lieves, to late stages. The chief reason why cells 

 of later cleavage stages cannot produce entire 

 ■embryos is because their power of cell division is 

 limited; hence enough cells cannot be produced 

 to form a complete embryo. (The very important 

 work of Crampton, showing that the develop- 

 ment of the isolated blastomeres of the gaster- 

 opod is, like that of the ctenophore, throughout 

 partial in character, is unaccountably left un- 

 mentioned by the author, though he cites other 

 articles which appeared in later numbers of the 

 same journal in which Crampton's paper was 

 published.) 



What, then, brings about the later differentia- 

 tion of cells if all the blastomere are totipotent ? 

 The author rejects the theory of qualitative 

 division of the nucleus ; he holds it impossible 

 also that the interaction of equivalent blasto- 

 meres should induce differentiation. That the 

 disti'ibution of yolk, etc., does not determine 

 differentiation is shown by the production of 

 normal larvse from that half of the echinoderm 

 ■eggs which contain no yolk. Professor Morgan 

 ■can only emphasize again that the experiments 

 on the ctenophore egg indicate that the factors 



in differentiation, whatever they may be, are 

 situated in the cytoplasm. What these factors 

 are, or even whether they may be placed in the 

 category of physico-chemical causes, we do'not 

 know. 



The remainder of the book, except the last 

 chapter, is taken up with a descriptive account 

 of the development of the frog's egg, from the 

 establishment of the germ layers to the moment 

 when the young tadpole emerges from the jelly 

 membranes. This account is chiefly abridged 

 from Marshall, and the figures are mostly copies 

 from the same author. Experimental work on 

 the later stages is not introduced, the remark- 

 ably interesting experiments of Born on the 

 grafting of parts of young tadpoles being too 

 recent to be included in the present volume. 



The last chapter is a brief review of researches 

 on the effects of different temperatures and dif- 

 ferent lights on development. An appendix 

 gives some hints on reagents, methods of preser- 

 vation, etc., and the whole is closed by an ex- 

 tended bibliography. 



Those chapters of the book (VII. -XII.) which 

 deal with the experimental work on the early 

 stages of development will be found a most 

 satisfactory presentation of the results in this 

 interesting line of work. The resume is ex- 

 tended enough to bring out all essential points, 

 is clearly written, fair and appreciative in its 

 account of opposing views, and the conclusions 

 set forth by the author are cautious and undog- 

 matic. 



The partially historical arrangement of the 

 material is advantageous in many respects. It 

 brings out with especial clearness the necessity 

 of caution in interpreting experiments on sim- 

 ple organisms, shows the fluctuations of 

 opinion in regard to the problems involved, 

 and aids essentially in understanding the pres- 

 ent status of investigation and opinion in re- 

 gard to these matters. On the other hand, this 

 arrangement brings the discussion of the ex- 

 perimental work out of relation with the rest of 

 the book. We should expect, from the title of 

 the work and the preface, that the descriptive 

 account of the embryology of the frog would 

 give the order of development of the subject. 

 Certain processes which require explanation 

 coming up in this account, it might be antici- 



