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. 
I lefaced 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 (German 
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 Yertebrata, 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, XYII to Tracheata, 
XVIII to Crustacea , and XIX to Pcecilopoda, 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 ; and 
the Crustacea from a Pliyllopod-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 antennae “ 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 chjetopoda j” or, if 
equivalent to appendages, they may correspond with the first pair of antennae 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 maxillse 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 tw r o groups ;” 
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 Gegenbaur and Lubbock, 
that in the JSphemeridce and Trichoptera the tracheal gills are modifications of 
wings (p. 339), the strong point that these organs are in several instances developed 
