1881. 239 



logists, as a rule, are more familiar with Comparative Morphology than with 

 Embryology, there is much in this volume well worthy of their consideration. 

 Prefaced by a short Introduction (pp. 1 — 13) explanatory of the science and aims of 

 Embryology, and describing the methods of animal reproduction in their general 

 plan, the first three chapters treat of the ovum and spermatozoon, giving a general 

 and special account of their formation, and of the development of the former up to 

 the period of its segmentation after impregnation. The distinction drawn by 

 Huxley between ova and pseudova is not adopted ; and " the term nurse (Grerman 

 Amme) employed for the asexual generations in metagenesis, may advantageously 

 be dropped altogether." 



The remainder of the volume is occupied by Systematic Embryology, Part I, 

 introduced by an explanation of what takes place in the ovum after segmentation 

 has been completed, a chapter being devoted to the Embryology of each class of 

 animals (excepting the classes of the Vertebrata, which are to form the subject of 

 Part II in the second volume), in addition to chapters relating to animals whose 

 affinities have not yet been exactly ascertained, the matter being discussed chiefly 

 from an histological point of view. The chapters of most immediate interest to 

 Entomological students extend from p. 316 to p. 452, and relate, XVII to Tracheata, 

 XVIII to Crustacea, and XIX to Poscilopoda, Pycnogonida, Pentatomida, and 

 Tardigrada, concluding with a summary of Arthropodan development. 



At pp. 451 — 452, Mr. Balfour advances embryological reasons adverse to the 

 opinion prevalent amongst Zoologists, that Tracheata and Crustacea are members of 

 the same phylum, and maintains that the Arthropoda have a double phylum, — the 

 Tracheata descended from a terrestrial Annelidan type related to Peripatus ; ar.d 

 the Crustacea from a Phyllopod-like ancestor. In support of these, he adduces 

 others based upon the anatomy of the animals in some particulars, pointing out 

 that " the Crustacean appendages are typically biramous, while those of the Tracheata 

 are never at any stage of development biramous." He does not admit that bi- 

 flagellate antennae in Pauropus (no other instances are cited) constitute an exception 

 to this rule, maintaining (p. 337) that antennse " can hardly be considered to have 

 the same morphological value as the succeeding appendages. They are rather 

 equivalent to paired processes of the prseoral lobes of the chsetopoda ;" or, if 

 equivalent to appendages, they may correspond with the first pair of antenna? of 

 Crustacea. This rule (that no appendages are biramous in Tracheata) having thus 

 been conclusively decided, it might be unkind to enquire, what is to be done with 

 the upper lnaxillee of such creatures as Lucanus, for example ? not to mention 

 Perlidce, &c, whose galea is jointless. 



The author justly alleges (p. 451) that " the similarity between the appendages 

 of some of the higher Crustacea and those of many Tracheata is an adaptive one, 

 and could in no case be used as an argument for the affinity of the two groups j" 

 whilst in respect of some other resemblances between them, they are due to " both 

 groups being descendants of Annelidan ancestors." The similarity of the compound 

 eyes in the two groups cannot, however, be explained in this way, and is one of the 

 greatest difficulties of the above view. 



In arguing against the supposition entertained by G-egenbaur and Lubbock, 

 that in the PJphemeridce and Trichoptera the tracheal gills are modifications of 

 wings (p. 339), the strong point that these organs are in several instances developed 



