Dec. 17, 1874] 



NATURE 



127 



"vital" ; and if at any time it should be shown, which is 

 well within the region of possibility, that they depend on 

 the manifestation of a force other than one of those with 

 which we are at present acquainted, the disciples of the 

 " vitalistic " school will have reason to exult over those 

 " physicists " who do not admit the existence of any yet 

 undiscovered mode of motion. As yet, the fact that one's 

 parents in their earliest days went through the same 

 changes as oneself is not considered a sufficient basis 

 for any logical hypothesis on the subject of the progressive 

 development of one's constituent elements. 



Again, since the time of Von Baer, the marvellous 

 parallelism which is so continually observed between 

 the various development-stages of living beings con- 

 siderably removed from one another in the scale of 

 zoological affinity, has made the study of embryology an 

 essential part of the science of Comparative Anatomy ; 

 in other words, the whole life-history of the individual, 

 and not only the period of maturity, is now known to be 

 necessary for our accurate comprehension of the pedigree 

 of the animal kingdom, in the same way that it may be 

 considered to reflect it. This conception has of late 

 borne fruit in the all-embracing hypotheses of Prof 

 Hffickel and Mr. E. Ray Lankester, as well as in the 

 new classification of the animal kingdom so recently pro- 

 mulgated at a meeting of the Linnean Society by Prof. 

 Huxley (Nature, vol. xi. p. loi). It may, however, be 

 mentioned that there is a limit to generalisation in this 

 direction ; for the theory of natural selection allows us to 

 assume that some of the forces which come into play to 

 produce variation in the individual, and therefore gene- 

 rally, may do so at the very outset of embryonic life ; 

 and, if they do so, differences from the ancestral type 

 may then appear in all the embryonic stages from the 

 commencement. Such a view of the question helps to 

 explain otherwise most involved subjects, such as the 

 existence of " gastrea; " of two entirely dift'erent types ; 

 the development of the notochord from different layers of 

 the blastoderm in different groups of animals ; and other 

 varying features of early embryonic life. 



These remarks all indicate how large a field is opened 

 up for the student of every branch of natural science by 

 the study of embryology ; and it is evident that before 

 any considerable progress can be made in any of the 

 many intricate problems involved, a minute acquaintance 

 with the fundamental facts of development is indispen- 

 sable. The work before us is the first systematic attempt 

 which has been made, in this country at least, to place 

 the whole subject on the required footing ; and in how 

 satisfactory a manner this has been accomplished will be 

 attested by all who have taken the opportunity of study- 

 ing it. When supplemented by the other two volumes 

 promised by the authors in their preface, it will form a 

 complete history of the most important changes known 

 to occur during the embryonic life of the different groups 

 comprising the animal kingdom. For a long time past 

 such a work has been a great desideratum. The mono- 

 graphs of different authors are scattered over a whole 

 library of books ; many who require to employ the known 

 results have but little time to investigate each sufficiently 

 to form a sound opinion of their own, and fewer still are 

 able to prosecute the somewhat special line of investiga- 

 tions on their own account. AH working biologists, 



therefore, owe much to Dr. Foster and Mr. Balfour for 

 the great care they have taken to sift the literature of the 

 subject, as well as for their independent investigations, 

 which add so considerably to our knowledge of a branch 

 of biology which has but little attracted the attention of 

 our own countrymen. 



To turn to the subject-matter of the work itself There 

 are advantages possessed by the hen's egg, found in no 

 other vertebrate embryo, which have led the authors to 

 take the liistoiy of the chick as the starting-point for their 

 subsequent descriptions. It is "the animal which has 

 been most studied, and the study of which is easiest and 

 most fruitful for the beginner." This must be evident to 

 anyone who has had the least experience. A chrono- 

 logical order is followed, in which the changes which 

 occur day by day, and sometimes even hour by hour, are 

 fully traced through the earlier days of incubation ; the 

 incidents of the later days being much more briefly sum- 

 marised, because they pertain more to the bird as a bird, 

 than to it as a member of the sub-kingdom Vertebrata. 



As above remarked, but little of the embryological work 

 which has been undertaken since the time of the illus- 

 trious Harvey has been conducted in this country ; it is 

 therefore not to be wondered at that we are far behind 

 the times regarding it. Many important points which for 

 some time past have been familiar to foreign investigators, 

 mostly German, are not sufficiently laid stress on, or are 

 omitted altogether, in our physiological treatises and text- 

 books. Among these may be mentioned the evanescent 

 nature of the " primary groove " in the mesoblastic layer 

 of the blastoderm, and its replacement by the "medullary 

 groove," from which alone, and .not from the former, the 

 spinal canal is subsequently formed. " The primary 

 groove, then, is a structure which appears early, and soon 

 disappears without entering directly into the formation of 

 any part of the future animal. Apparently it has no 

 function whatever. We can only suppose that it is a 

 rudiment of some ancestral feature," remark our authors. 



The much-debated subject of the development of the 

 blood-vessels and corpuscles is entered into in detail, and 

 fresh investigations by one of the authors are recorded, 

 which agree in many respects, as they remark, with those 

 of Remak and Klein. The vessels are shown to be 

 formed by the union of processes sent out from the meso- 

 blast cells of the pellucid area. The nuclei of these cells 

 enlarge and break up into numerous small ones, the 

 majority of which acquire a red colour, and become con- 

 verted into blood- corpuscles ; whilst the rest, changing 

 into a spindle-shaped form, develop into the synovial 

 lining of the blood-vessels. 



Another point of special interest is the development of 

 the permanent vertebral column. As all know, the proto- 

 vertebrje are developed at the sides of the notochord, 

 with a neural arch attached mainly to the posterior end 

 of each ; whilst the root of a spinal ner\-e occupies the 

 anterior portion. " On the fourth day the transparent 

 lines marking the fore and aft limits of the protovertebrs 

 are still distinctly visible. On the fifth day, however, 

 they disappear, so that the whole vertebral column 

 becomes fused into a homogeneous mass whose division 

 into vertebrae is only indicated by the series of ganglia. 

 This fusion ... is quickly followed by a fresh segmenta- 

 tion, the resulting segments being the rudiments of the 



