NA TURE 



203 



THURSDAY, APRIL 13, 1911. 



PROBLEMS OF SEXUAL AND ASEXUAL 

 REPRODUCTION. 



Lehrbuch der vergleichenden Entwickluugsgeschichte 

 der wirbellosen Thiere. By Prof. E. Korschelt and 

 Prof. K. Heider. Erste und Zweite Auflage, All- 

 gemeiner Theil, Vierte Lieferung. Erste Hiilfte. 

 Po. 167-470. Price 7.50 marks. Zweite Halfte. 

 Pp. 471-896. Price II marks. (Jena: Gustav 

 Fisclier, 19 10.) 



EAIBRYOLOGISTS wlio are already acquainted 

 with the " Lehrbuch der vergleichenden Ent- 

 \\ icklungsgeschichte der wirbellosen Thiere " — and 

 \\ hat embr3ologist is not? — will welcome with delight 

 appearance of these further instalments of the 



Vllgemeiner Theil." No pains have been spared in 

 liie preparation of these two volumes, and authors 

 and publishers alike are to be congratulated on their 

 iichievement. 



In the first the student is introduced to the 

 earliest important phase in the differentiation 

 of the sexually produced individual organism — the 

 formation of those " elementary organs " long since 

 known as the layers of the germ. An admirably 

 lucid account is given of the various types of " gastru- 

 lation," or endoderm formation, and this is followed 

 by a description of the development of the mesoderm. 

 The latter part of the treatise is devoted to the study 

 of the peculiarities of these processes in certain special 

 groups, Sponges, Arthropods, and Chordates (Cephalo- 

 chorda and LVochorda). 



It seems almost ungenerous to criticise where the 

 general level of attainment is so high, but we confess 

 that we should have liked to see the Vertebrates in- 

 cluded in the last chapter— the method of closure of 

 the blastopore, its relation to the chorda and neural 

 plate are so similar in all these forms — and it is cer- 

 tainly a pity that no attempt has been made to deal 

 with the derivation of the Amniote from the conditions 

 found in Anamnia — a problem long the despair of 

 embryologists, but now happily solved with the help 

 of the Gymnophiona. And we might venture to re- 

 mark perhaps that Fig. 154A, on p. 248, is incorrect 

 in not exhibiting the bilateral structure of the egg, 

 due to the formation of the gray-crescent on fertilisa- 

 tion. These, however, are matters that hardly mar 

 the excellence of the book. 



The second volume makes even more fascinating 

 reading. One by one we are shown, in the groups 

 of the animal kingdom, the details of the mechanism 

 by which a new individual is produced from a bud, 

 processes simple enough in some cases, but often 

 presenting the most amazing complexities, which 

 reach their acme in the Ascidians, while at the same 

 time they defy — notably in Polyzoa and Ascidians 

 again — the rules observed in sexual reproduction, and 

 challenee the observer to frame any formula' which 

 will embrace the infinite variety of their behaviour. 



Hut description is but the beginning of embryology, 

 as of any other science, and the baffling problems 

 presented by the germinal lavers will assuredly only 

 NO. 2163, VOL. 861 



yield, if at all, to the experimental method. The 

 results which that method has obtained must indeed 

 form the starting point of any discussion, the basis of 

 any theor\' of the part played by these " elementary 

 organs " in development. 



The old germ-layer hypotheses, which all more or 

 less trace their lineage back to Haeckel's famous 

 " Gastraea-Theorie," assume that the stage at which 

 these sets of cells are segregated is of phylogenetic 

 significance, reminiscent of some bygone ancestry, 

 and that there is a general agreement not only in 

 the way in which the layers originate, but also in 

 the structures to which they eventuqjly give rise, as 

 is, of course, necessary for the hypothesis. 



And though the authors of this treatise very properly 

 define the germ-layers as the rudiments of definite 

 body tissues in normal development, they are yet 

 concerned to attempt some defence of a position which 

 is no longer tenable. 



Even in sexual reproduction there are too many 

 refractory facts : thus, a two-layered condition is not 

 necessarily a separation of endoderm from ectoderm 

 (Placental Mammals, Cestodes, Phylactolaemata), the 

 gut of Coelenterates is formed in various wa5's, and 

 so in Vertebrates the roof and floor of the archenteron 

 plav varying parts in the formation of that organ, 

 while in those groups in which the egg segments 

 according to the "spiral" type, it is by no means 

 necessary that a cell or cell-group which has a de- 

 finite place and time of origin in the cleavage system 

 should invariably give rise to the same larval or 

 embryonic part, though, it is true, this usually occurs. 

 The cell ^d, for instance, is not always mesoblastic, and 

 even when it does give rise to the mesoblastic bands 

 the destiny of those structures may be variable. In 

 Annelids, the ccelom is developed in them, in certain 

 Mollusca they break up into connective tissue while 

 the (coelomic) pericardium has another (ectodermal) 

 source. 



These are only a few instances. When, however, 

 we turn to the conduct of the layers in budding 

 matters are infinitely worse. In Polyzoa the whole 

 of the alimentary canal is of ectodermal origin'; in 

 Turbellarians nervous system and pharynx are meso- 

 dermal; in Annelids the pharynx is endodermal, all 

 these organs being in sexual reproduction developed 

 from the ectoderm. In Cephalodiscus the gut is ecto- 

 dermal instead of being derived from the parental 

 endoderm. In the Tunicata these anomalies attain 

 the perfection of irregularity. The nervous system 

 mav be ectodermal (Botryllidae), endodermal (other 

 Ascidians), or mesodermal {Pyrosonta, Salps, DoHo- 

 lidae). The atrium may be ectodermal (Botryllidae), 

 endodermal (other Ascidians), or mesodermal {PyrO' 

 soma, Salps), while the pharynx may be ectodermal 

 (Botryllida;, Doliolidse), or endodermal (other Asci- 

 dians). 



These facts, as the auihi)rs admit, nay insist, are 

 indisputable, and absolutely irreconcilable with the 

 requirements of the ordinary germ-layer theory. 



An effort is indeed made to save the situation by 

 minimising the discrepancies that occur in the course 

 of sexual reproduction, and removing the facts of 



