1880. | Recent Literature. - 663 
derms; the Protozoa not being considered. The author has evi- 
dently adopted Lankester’s classification, as the Tunicata are not 
referred to in this volume, and will, undoubtedly, be taken up in 
the vertebrate volume 
The introduction treats of the development of the ovum and 
spermatozoon, the maturation and impregnation of the ovum, and 
its mode of segmentation. These subjects are treated in a clear 
way, with an abundance of illustration, and this portion will be 
found as useful as any part of the book, since the phenomena 
have been traced by but a few and are rather difficult to com- 
prehend. 
The introduction to systematic embryology refers to the mode 
of origin of the germinal layers, and the gastrula, both by invagi- 
nation and by delamination, the figures very clearly showing these 
different modes of development of the germinal layers and primi- 
tive digestive cavity. We observe that Mr. Balfour uses the con- 
venient terms morula and gastrula to express the two earlier 
stages in the life history of most animals, though he is unable to 
perceive a true gastrula condition in the Tracheata. 
The remainder of the work is devoted to special accounts of 
the mode of development of such types of the different classes as 
have been studied. These include the Dicyemidæ and Orthonec- 
tide, Porifera, Coelenterata, Platyelminthes, Rotifera, Mollusca, 
Polyzoa, Brachiopoda, Cheetopoda, Discophora, Gephyrea, with 
references to the aberrant types of Chzetognatha, etc., Nematel- 
minthes, ending with the Tracheata, Crustacea, Pcecilopoda, 
Echinodermata and Enteropneusta, in the order here given. 
This order represents the author’s views as to the classification, 
or what is but another name for it, the phylogeny of the inver- 
tebrates. We are gratified to see that the author assigns the 
_ Sponges to a group by themselves, while the Mollusca have the 
Same rank assigned to them as to the Polyzoa or Brachiopoda. 
We scarcely see why the Echinoderms are placed so far away 
from the Ccelenterates and worms. laa 
_At the end of nearly every chapter, after treating of the indi- 
vidual development, that of the organs is discussed, and finally, as 
in that on the insects (Tracheata), we are treated to a rehearsal of 
the general mode of formation of the layers an embryonic 
envelopes, with a full bibliography. As regards the latter it 
would be much more convenient to the student, especially the be- 
ginner, if the works had been cited in the order of publication, so 
as to induce one to study the history of the literature. After 
treating of the Tracheata and Crustacea, and Poecilopoda, Pycno- 
gonida, Pentastomida and Tardigrada, an excellent summary of 
Arthropodan development is given. : i 
he theoretical portion relating to phylogeny is kept apart, 
and is discussed in a generally conservative and judicious way; 
the bearings of paleontology, though of primary importance as 
