xii PREFACE TO THE SECOND EDITION 



to pass by, or dismiss with very brief mention, many works to which 

 space would gladly have been given. 



Recent research has yielded many new results of high interest, 

 conspicuous among them the outcome of experiments on cell-division, 

 fertilization, and regeneration ; and they have cleared up many special 

 problems. Broadly viewed, however, the recent advance of discovery 

 has not, in the author's opinion, tended to simplify our conceptions 

 of cell-life, but has rather led to an emphasized sense of the diversity 

 and complexity of its problems. " One is sometimes tempted to con- 

 clude," was recently remarked by a well-known embryologist, " that 

 every o.^^ is a law unto itself ! " The jest, perhaps, embodies more 

 of the truth than its author would seriously have maintained, express- 

 ing, as it does, a growing appreciation of the intricacy of cell-phe- 

 nomena, the difficulty of formulating their general aspects in simple 

 terms, and the inadequacy of some of the working hypotheses that 

 have been our guides. It is in the full recognition of such inade- 

 quacy, when it exists, and of the danger of hasty generalization in a 

 subject so rapidly moving as this, that our best hope of progress lies. 



My best thanks are again due to many friends for helpful criti- 

 cism, suggestion, and other aid ; and I am indebted to Professor Ulric 

 Dahlgren for the beautiful preparation imperfectly represented by 

 Fig. 59 (from a direct photograph); to F. Emil, E. M. Van Harlin- 

 gen, and Dr. G. N. Calkins, for aid in the preparation of new illus- 

 trations ; and to Messrs. Ginn & Co. for the electrotypes of Figs, ii, 

 12, and 1 88, from the Wood's Holl Biological Lectures for 1899. 



Columbia University, 

 December 7, 1899. 



Postscript. — Of especial importance for some of the discussions in Chapters I., V., and 

 VII. are Fischer's extensive work on protoplasm (see Literature, I.) and Strasburger's new 

 researches on reduction (see Literature, V.), both received while this volume was in press 

 and too late for more than a passing mention in the text. 



March, 1900. 



