October i, 1903] 



NATURE 



523 



EXPERIMENTAL EMBRYOLOGY. 



Lehrbucli der vergleichenden Entwicklungsgeschichte 

 der U'irbellosen Thiere. By Profs. E. Korschelt and 

 K. Heider. Allg". Theil, Erste Lief., Erste und Zweite 

 Auflage. Pp. x + 538. (Jena: Fischer, 1902.) 

 Price 14 marks. Zweite Lief., Erste und Zweite 

 Auflage. Pp. 539 to 750. (Jena : Fischer, 1903.) 

 Price 5.50 marks. 



ZOOLOGISTS who are already acquainted with the 

 " special " part 01 Profs. Korschelt and Heider's 

 " Comparative Embryology " will have been anxiously 

 looking forward to the publication of the present 

 volume; we are sure that they will in no wise be dis- 

 appointed. At present we have only a first instalment, 

 but even this contains an enormous amount of matter, 

 including, as it does, a review of all the recent work 

 on the physiology of development, besides a complete 

 history of the sexual cells. 



The latter portion, we may as well say at once, 

 should have come first. Logically, the phenomena of 

 what Roux has called " Vorentwickelung " are more 

 closely related to descriptive than to experimental 

 embryology ; and if the order of the first and second 

 portions had been reversed, the authors would have 

 been able to include under a common discussion the 

 kindred problems of ontogeny and heredity. 



Of this second portion we have no space to treat at 

 length. It must suffice to say that the student will 

 find here an excellent resumd of all that is known on 

 the structure, maturation, and fertilisation of the germ- 

 cells. Criticism is hardly called for ; but the definition 

 of the mammalian placenta (p. 292) is out of date, and 

 we should have liked to have seen a less fragmentary 

 account of the maturation phenomena in plants. On 

 the other hand, the difficult subject of maturation is 

 treated with remarkable lucidity, while the attitude of 

 the authors towards the vexed questions of qualitative 

 reduction, and, in the next chapter, the individuality 

 of the centrosome, is admirable in its judicial im- 

 partiality. 



By far the most important part of the book, how- 

 ever, is the first section — that dealing with the work 

 of the new school of experimental embryologists. The 

 problems at issue are sharply defined in an introductory 

 preface. As the authors rightly remark, ontogeny 

 consists of a series of changes in which every stage is 

 — in the strictest sense of the word — a cause of that 

 which immediately follows. The business of the ex- 

 perimenter is to analyse the phenomena, to deter- 

 mine what is due to external, what to internal factors, 

 and, in respect to the latter, how much is attributable 

 to the initial structure visible or invisible of the ovum, 

 how much to the mutual interaction of the parts that 

 are successively developed. 



With this object in view the ground is first cleared 

 by a discussion of the external factors, beginning, 

 quite rightly, not only from a logical, but from a 

 historical point of view, with the pioneer work 

 of Pfliiger on the influence of gravity on the 

 segmentation of the frog's egg. An account of the 

 subsequent, and consequent, work of Born, Roux and 

 Hertwig naturally follows. Next are described the 



NO. 1770, VOL. 68] 



effects of heat, light, and physical and chemical 

 changes in the gaseous and liquid environment, and 

 lastly, a little out of their proper place we think, the 

 few experiments that have been made to determine 

 the influence of electricity and magnetism, and of 

 mechanical disturbances on the course of development. 



It is a pity that the authors have not introduced 

 at this point a critical summary of the results. It is 

 of the first importance to decide whether these external 

 conditions constitute a series of " specific " or merely 

 " indifferent " causes. Hertwig's artificial production 

 of monsters by heat and salt solutions would have 

 made an apt text for an interesting essay on " Abhan- 

 gige Differenzirung," and would have served to carry 

 on the reader to the next chapter, " Das Determin- 

 ationsproblem," in which we are taken straight to the 

 heart of the " Streitfrage " of modern embryology. 



While the restoration of the eighteenth century doc- 

 trine of preformation to a prominent place in embryo- 

 logical literature dates from His's theory of " Organ- 

 bildende Keimbezirke," the attempt to gauge its 

 worth experimentally begins with Roux's work on the 

 production of half-embryos from a single blastomere 

 of the frog's ovum. Roux's results, or rather his in- 

 terpretation, were wholly in favour of this doctrine; 

 their value has, however, been diminished by Hert- 

 wig's criticism and Herlitzka's work on the newt. 

 The Amphibia, indeed, together with Amphioxus, the 

 Teleostei, and the Coelenterata, stand, so far as the 

 " regulative " capacity of their ova are concerned, at 

 one end of a series, at the other extreme of which are 

 forms, the Ctenophora and Mollusca, the isolated 

 blastomeres of which are incapable of developing into 

 anything but partial larvae. The intermediate position 

 is occupied by the Echinoderms and Ascidians ; here 

 the segmentation of such blastomeres is partial, but a 

 whole larva is ultimately formed. Any general theory, 

 therefore, of the necessary predetermination of the parts 

 of the organism in the cytoplasm of the ovum is out 

 of the question. A similar criticism, based on the 

 pressure experiments of Driesch (Echinus) and Hert- 

 wig (Rana), is applicable to the nucleus, and, of 

 course, cuts at the root of the " Mosaik-Theorie. " 



The failure of the attempt to demonstrate a pre- 

 formed, though invisible, structure in the ovum 

 throws us back on epigenesis, and compels us to search 

 for the internal causes of ontogeny in the mutual inter- 

 action of the parts as they are formed. To deduce 

 such interaction, however, from the known functions 

 of cells is a very different matter; but such facts as 

 are significant for the purpose are brought together 

 in "the third chapter under the heading of " morpho- 

 genetic cellular processes." 



The general discussion of the whole problem is re- 

 served for a separate appendix. The authors display 

 a commendable caution in reviewing the theories of 

 Weismann, Hertwig, and Driesch, This caution, 

 indeed, is characteristic of the whole book, and will 

 certainly win the approbation of every embryologist 

 who is content to say with the authors, *' wir werden 

 die Speculation nie entbehren konnen, aber es wird die 

 Aufgabe sein, das ihr zu Grunde liegende Beobacht- 

 ungsmaterial moglichst zu erweitern." 



