60 BihliograpMcal Notices. 



phenomena of the geographical distribution of animals and their 

 succession in geological time, whatever theory of their production 

 we may adopt, are generally in accordance with the results of a 

 theoretical genetic relationship. With the prevalence of such ideas 

 a new significance was given to the phases tlirough which animals 

 pass in their progress to their perfect form ; and it is hardly to he 

 wondered at that the study of embryology, taken in its broadest 

 sense, began to be followed with a zeal and energy of which we had 

 no previous conception. The zoological laboratories which have been 

 established in several favourable situations ofiered every facility for 

 carrving on the most minute and elaborate investigations ; individual 

 students of course under such circumstances experienced an increased 

 stimulus to exertion ; and the result during the last fifteen years has 

 been a perfect deluge of memoirs, of greater or less merit, treating 

 of the developmental history of animals. 



It is to the sifting and summarizing of this vast mass of material, 

 aided by his own investigations, that Mr. Balfour has devoted an 

 enormous amount of labour, the outcome of Avhich is the volume 

 whose title stands at the head of the present article, and for which 

 all zoologists certainly owe him a deep debt of gratitude. The 

 introduction of new ideas in connexion with embryonic development 

 has resulted in such a multiplication of technical terms that many 

 naturalists who have not made embryology their study must often 

 find it difficult to understand the precise nature of the statements 

 made and the arguments used in the discussion even of questions of 

 systematic zoology ; and to these Mr. Balfour's book will be an in- 

 expressible boon. But this is the lowest point of view from which 

 we can estimate its usefulness. As a philosophicah summary of the 

 results of embryological investigation it must be quite as highly 

 appreciated. 



Mr. Balfour commences with an Introduction, in which, after 

 indicating the general purpose and scope of his work, he briefly 

 describes the phenomena of reproduction and its different modes. 

 He then proceeds to describe the nature and development of the 

 ovum and spermatozoon, the maturation of the former and its im- 

 pregnation, and the subsequent changes produced by segmentation 

 «S:c. up to the period of the formation of the germinal layers. The 

 general statements are illustrated by references to the pheno- 

 mena presented by certain groups ; and the whole constitutes an 

 admirable sketch of the process of ovular development in the animal 

 kingdom. 



These chapters are followed by the section which constitutes the 

 body of the work, systematic embryology, in which the author, after 

 describing the general phenomena resulting in the formation of the 

 germinal layers, and the broad differences in the mode in which this 

 result is brought about, proceeds to describe seriatim the character- 

 istics of embryonic development in all the great groups of the animal 

 kingdom. Criticism of such work would be out of place ; we can 

 only say that, so far as we can see, all the most recent literature of 



