March 29, 1888] 



NATURE 



507 



embryologist who bends himself to the requirements of 

 the stereotyped curricuhim of human anatomy as 

 demanded alike by German and English directors of 

 medical education, necessarily abandons more or less the 

 consistent scientific treatment of a branch of human 

 knowledge. There is a conventional embryology of the 

 medical school just as there is a conventional anatomy, 

 histology, and physiology, and as there would be a 

 " medical " chemistry, physics, and biology, and a 

 " medical " alphabet, if some professional men in London 

 were to have their own way. Prof Hertwig has suffered 

 somewhat by submission to these demands. The second 

 half of his work consists chiefly of a description of the 

 development of the various organs, and would more 

 appropriately find its place in that dullest, but most neces- 

 sary, of treatises — a text-book of human anatomy. The 

 first part, however, is not open to this objection, and even 

 in dealing in the usual way with the development of the 

 organs of the human body in the second half of his work, 

 Prof. Hertwig has managed to bring in a good deal of 

 that scientific interest which is briefly indicated by 

 Goethe's word " morphology." Nevertheless the detach- 

 ment of the consideration of the mode of origin of the 

 organs of the human body from that of their adult struc- 

 ture and of the structure and development of the same 

 organs in other animals is, in our opinion, an antiquated 

 and mistaken usage, which we are sorry to find so able 

 an author as Prof. Hertwig constrained to follow. 



In his first part Prof. Hertwig adopts a more general and 

 truly scientific treatment, and does not distinctly aim at sup- 

 plementing the work of the topographical anthropotomist. 

 His first chapter is a description of the se.xual products, 

 and his consideration is by no means limited to the ovum 

 and spermatozoon of the human species. A comprehen- 

 sive, though brief, account of the subject with reference to 

 various recent writers is given, and the classification of 

 animal eggs proposed by Balfour is adopted, viz. alecithal 

 telolecithal, and centrolecithal. 



The maturation of the egg and its fertilization are 

 treated in the second chapter, with special reference to 

 the Echinoderma and other Invertebrata, where it has been 

 possible to study this subject with advantage. A third 

 chapter treats of the process of egg-cleavage — the forma- 

 tion by division of the first embryonic cells ; a fourth, of 

 the general principles of development — the latter decidedly 

 brief and undeveloped to a degree which is disappointing. 

 Then we come to a chapter on the development of the 

 two primary germ-layers — or on the gastrtea theory, as 

 Prof. Hertwig puts it— in which the apparent differences of 

 development of these two layers in various Vertebrata are 

 considered and reconciled, numerous illustrations being 

 introduced into the text, of which a larger number are 

 taken (with ample acknowledgment both in the text and 

 in the special titles of the cuts) from the " Comparative 

 Embryology " of the late Prof Balfour than from any 

 other source. 



The development of the two (parietal and splanchnic) 

 middle germ-layers (coelom theory) is the next subject of 

 consideration, and is elucidated by a consideration and 

 figures of the process in Sagitta, Amphioxus, Triton, 

 the Mole, &c. The seventh chapter, on the history of the 

 germ-layer theory, is an able and fair statement of the 

 history of embryological doctrine such as every student 



should be familiar with, and it brings us to the special 

 Hertwigian doctrine of pseudocoel and mesenchyme. 

 The latter is further placed before the reader in the 

 chapter on the development of connective substance and 

 blood. In dealing with the special subject of this book 

 Prof Hertwig has no occasion to enter upon the question 

 of the pseudocoel— a theoretical conception which, in our 

 opinion, is unnecessary, and not supported by even plausible 

 evidence. The use of the term " mesenchyme " for those 

 cell-elements of the mesoblast layer which lie below the 

 layers immediately bounding the ccelom, and which give 

 rise to connective-tissue and to blood, is, in our opinion, 

 inadvisable. The distinctness which is implied in the use 

 of this term is not, it seems to us, in accordance with the 

 facts of embryology, and we think that embryological 

 appearances may be more correctly stated without intro- 

 ducing the conception of a distinct " mesenchyme," and 

 without postulating a " pseudocoel " in certain Inver- 

 tebrata, and by adhering to what we may call the 

 " uniform itarian " system, which seeks to explain " pseu 

 docoel" and "mesenchyme" as a special modification of 

 the normal "coelom" and "mesoblast" respectively — 

 these modifications arising independently under given 

 mechanical conditions in various developmental histories. 

 At the same time, it must be admitted that the attempt 

 to assign a special importance and genetic persistence to 

 " mesenchyme " on the part of the brothers Hertwig has 

 led them to bring many important embryological facts 

 into clear view. The speculations of His as to parablast 

 and archiblast are finally rejected, and a comparatively 

 harmless, though, it would seem, superfluous, theory 

 replaces it. 



In the chapter on the primitive segmentation of the 

 body, we come to closer quarters with the ultimate aim of 

 the treatise, viz. the human embryo; and this is followed 

 by chapters on the " Formation of the External Form," 

 and on the " Egg-membranes of Birds and Reptiles," and 

 on the " Egg-membranes of Mammals." These are well 

 illustrated by some of the best amongst already familiar 

 woodcuts (from Balfour, Kolliker, and Turner), and by a 

 coloured plate. At length, in the last chapter of the first 

 portion of his work. Prof. Hertwig brings us to the human 

 interest which has been the motive of all the previous 

 exposition. Here are discussed the " Human Egg-mem- 

 branes." The medical student is at last rewarded for his 

 patience in wading through the chapters of a scientific 

 treatise, and has the embryo of Allen Thomson, of Coste, 

 and of Krause made clear to him. An excellent account 

 of the structure of the human placenta, accompanied by 

 many woodcuts and by a coloured plate, is given. 



Then follows the second " Abtheilung," with its 

 necessarily uninteresting and disjointed account of the 

 development of organs. Whilst recognizing the value, 

 and, in many features, the originality, of this part of the 

 work, we must insist that even so accomplished a writer 

 as Prof. Oscar Hertwig could only do justice to this sub- 

 ject by treating it as part of a comprehensive work on 

 the morphology of Vertebrata, and this the space at his 

 command has not allowed him to attempt. The student 

 will, however, find clear expositions and the latest in- 

 formation on the development of the organs of Verte- 

 brata, with a special reference to the higher Mammalia 

 or man. As an example of the thoroughness with which 



