NATURE 



451 



THURSDAY, JUNE 17, 1909. 



EXPERIMENTAL EMBRYOLOGY. 

 Experimental Embryology. By J. W. Jenkinson. Pp. 

 viii + 341. (Oxford: At the Clarendon Press, 1909.) 

 Price 125. 6d. net. 



EMBRYOLOGY as a branch of science is usually 

 talven as dating back to the publication, by 

 Caspar Friedrich Wolff, of the " Theoria Genera- 

 tionis," in 1759. Experiment, as an aid to the study, 

 is of more recent date, and may be considered as com- 

 mencing with Prof. Wilhelm Roux in 1883, twenty- 

 six years ago. The coming-into-being of any living 

 thing is a cycle beginning at a certain point ana 

 ending at a similar one, the starting-point of a new- 

 round. Primarily the studies of the embryologist are 

 concerned with the elucidation and explanation of the 

 phenomena included within this. Wolff tried to 

 demonstrate, in the instance of the chick, how from 

 an apparently undifferentiated mass, what we should 

 now term the cleavage-products of the egg, part was 

 gradually added to part, much in the manner that any 

 day one may witness the building of a house from a 

 heap of materials. He thus founded what is known 

 as the doctrine of epigenesis. .AiS Dr. Jenkinson 

 remarks, this theory was " tacitly accepted by all the 

 great embryologists of the nineteenth century — 

 Pander, von Baer, Reichert, Bischoff, Remak, von 

 Kolliker, Kowalewsky, Haeckel," and " the epigenetic 

 idea continued to control the progress of research." 



As initiated by Prof. Roux, the experimental study 

 of embrj-ology is based on a modification of the doc- 

 trine of epigenesis, little or no consideration being 

 paid to any rival theory of development. Following 

 on the work and theories of Wilhelm His, Roux raised 

 " what in His's hands had been merely a principle to 

 the rank of a theory, the ' Mosaik-theorie,' or theory 

 of self-differentiation," under which " every separatelv 

 inheritable quality of the body has its own representa- 

 tive in the germ." 



In this there has always been included implicitlv 

 the belief that it was the main, if not the sole, task of 

 the fertilised egg to give rise to a new embryo. In 

 this way the importance of " the embryo " in the 

 embryological mind has been greatly exaggerated, and 

 little or no weight has been laid upon other pheno- 

 mena of the life-cycle prior to its appearance. As 

 research so often has revealed in recent years, the cycle 

 from egg to egg is possibly, even probably, nothing 

 like so simple as the upholders of epigenesis in the 

 nineteenth century and earlier believed and taught. 

 It appears, therefore, of prime importance that, prior 

 tq operative interference with, say, the egg-cleavage 

 of any particular form, a complete knowledge of the 

 normal life-cycle be obtained. How necessary this is 

 to the investigator is evident from the consideration 

 that in many of the best investigated cases most of 

 the cleavage-products are concerned, not in forming 

 by epigenesis portions of an embryonic body, but in 

 producing structures of an evanescent character, un- 

 represented in any way in the adult animal, or even 

 in the embryo. 



NO. 2068, VOL. So] 



Then, as a perusal of the present ably-written book 

 reveals, the experimental embryologist seldom has 

 anything of the nature of an embryo before him. He 

 mav, and often does, speak of the Pluteus of a brittle- 

 star, or the Bipinnaria of a starfish, indifferently as a 

 Inrva or as an embryo. But there is no homology 

 between the Pluteus and the brittle-star ; neither as a 

 whole nor as to its parts is the one converted or 

 " metamorphosed " into the other, as mythologically 

 Jupiter turned himself into a bull. In short, direct 

 development is also assumed. 



Modern forms of the doctrine of evolution or unfold- 

 ing are usually supposed to be the " Mosaik-theorie " 

 of Roux and the " Germplasm-theory " of Weismann. 

 The former has much in common with epigenetic 

 doctrine, and the latter, with the ids, determinants. 

 &c., really only transfers the powers of the builder, 

 epigenesis, to the cell-nucleus of the fertilised egg. 

 There is a theoiry of development, not mentioned in 

 the present book, and which had its most recent 

 advocacy in the last presidential address to the 

 British Association for the .Advancement of Science. 

 This is Ewald Hering's theory of (unconscious) 

 memory of germ-cells (1876), which in recent years 

 has been elaborated by Richard Semon (and by the 

 writer). This theory of "Die Mnemc," to use 

 Semon -s term, along with an actual contmuity ol 

 o-erm-cells from generation to generation, would 

 appear to be in better accord with the facts of animal 

 development than that of epigenesis. Unlike the 

 latter, it explains, for instance, how from a single egg 

 two embryos (identical twins), or even as many as 

 eleven (an armadillo, Praopus hybridiis), may arise. 



If in 'first principles investigation be based er- 

 roneouslv, as experimental embryology would appear 

 to have been hitherto, it cannot be wondered that the 

 results have been discordant, or that no really funda- 

 mental advances have been made. There is still 

 another point to be emphasised, that is, how difficult 

 even impossible, it is in any experimental study of 

 development to be sure what has actually been done 

 in anv operation. In a sense the results always border, 

 on pathology, although a recent writer. Prof. H. H. 

 Wilder, has given good reasons for the opinion that, 

 as a rule, monsters, as they occur in nature, cannot 

 be produced experimentally, at all events by 

 mechanical means. It is also a feature of practically- 

 all researches in experimental embryology that the 

 organisms dealt with never revert to the normal, never 

 resume or continue for any appreciable period the 

 normal life-cycle. 



The difficulties in the way of estimating the results, 

 as well as of determining what has actually been done 

 in the operations, throw light on other conclusions, 

 such as Driesch's speculations on neo-vitalism, and on 

 mechanical theories of the nature of life. 



The investigations hitherto made in experimental 

 embryology and their results, as given with an excel- 

 lent critical discussion in this book by Dr. Jenkinson, 

 remind one of " die Sieben Weltnithsel " of Emil du 

 Bois Reymond. No doubt the study is a fascinating 

 one, for the unexpected is always happening in it, but 

 it is questionable whether any investigations yet made' 



