viii PREFACE. 



hut lie is encouraged, and to a certain extent trained, in the healtliy use of 

 liis si^eculative faculties ; in fact the one great method by which new know- 

 ledge is attained, whether of little things or of big things — the method of 

 observation (or experiment), directed by speculation — becomes the con- 

 scious and distinctive characteristic of his mental activity. Thus we may 

 claim for the study of Comparative Anatomy, as set forth in the present 

 work, the poAver of developing what is called "common sense" into the 

 more precisely fixed " scientific habit " of mind. 



I have made no notes nor additions of any kind to the original text, 

 with the exception of a few references to English Avorks likely to be useful 

 to the English student. These additions are indicated by brackets. 



Whilst the work is thus presented to the reader precisely as its author 

 designed that it should be, there can be no objection to the introduction 

 in this place of a few remarks suggested by the fact that this English 

 translation is intended for the use of English students, and that it is 

 therefore desirable, in order to prevent confusion and perplexity, to jjoint 

 out certain statements of fact, or of interpretation of fact, in Avhich Professor 

 Gegenbaur differs Avidely from authorities usually foUoAved in this country. 

 I shall, moreover, refer to some recent additions to knowledge published 

 since this Avork left Professor Gegenbaur's hands. It Avill be rmderstood 

 that the foUoAving paragraphs are intended as a supplement necessitated 

 by the special objects of this translation, and are by no means to be 

 regarded as conceived in the spirit of criticism or discussion, Avhich Avoiild 

 assuredly ill befit a Avriter Avho is making knoAvn to a ncAV audience the 

 teachings of a master to Avhom he is deeply indebted. 



Nuclei of Cells. — In the first place, it seems necessary to notice 

 that, Avhilst the last German edition of this AVork Avas in the press, very 

 important additions to our knoAvledge of the nucleus of organic cells or 

 plastids Avere being made. Though these inA'estigations are not yet complete 

 they tend to modify AA'hat is said concerning the nucleus on pages 15 and 

 16. The student is referred to an article by Mr. Priestley in the Quart. 

 Journ. Microsc. Science, vol. xvi. (1876), for an account of the observations 

 of Auerbach, Strasburger, Hertwig, and Van Eenedex, and to jDart iii. 

 of the same Journal, vol xviii. (1878), for original observations on the same 

 subject by Dr. Klein. 



Reproduction of Infusoria. — A most important modification in the 

 current A'icAvs as to the reproduction of the Infusoria has resulted from 

 the same line of study as that just mentioned, Avhen carried into the 

 domain of unicellular organisms. 0. Butschli and Engelmann have shoAvn 

 that Ave are not at present in a position to assert that the process of con- 

 jugation in the Infusoria is followed by a production of spores (see § 70). 

 It results from their investigations that conjugation in the Infusoria is 

 attended by a definite breaking-up of the nucleus and so-called nucleolus 

 (paranucleus) of the conjugating individuals ; but that the conjugating 



