ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 767 
evolution theories appear to the recorder somewhat futile, and his 
philosophic cautions familiar, we gladly give his protest due mention. 
As he says, “ La verita non pub perire dunque di che temere? ” 
Principles of Skeleton-forming.* — Dr. F. Dreyer continues his 
“ essay towards a mechanical explanation of organic structures ” by a 
discussion of the skeletons of Rhizopods, Sponges, and Echinodermata. 
In the present part of his memoir he begins with the tetraxial type in 
Sponges, Echinoderms, and Polycystina. The fact that the tetraxial 
type occurs independently in several groups suggests the conclusion 
that it is conditioned not by the specific vitality of the organisms, but 
by similar inorganic factors. Its occurrence in different materials 
suggests independence of protoplasmic forces and dependence on the 
mechanical influences of the environment. Vesicular bodies exhibit a 
tetraxial skeletal structure as the result of definite physical conditions — 
a discussion of which will be found in Dreyer’s essay. The skeletal 
substance is secreted by and in the living matter; it arises by the 
calcification, or silicifying, or cornification of organic parts, which are 
dependent on the conditions of vesicular tension and tend to a tetraxial 
arrangement. The tetraxial skeletons of Sponges, Echinoderms, and 
Polycystina thus depend on the physical 'conditions of vesicular 
tension. 
The fourth chapter of Dr. Dreyer’s essay deals with mosaic-shells, 
composed by the apposition of small parts. Here the mechanical 
conditions are obviously more complex, and some may think that the 
facts are somewhat beyond the author’s theory of “ Bildungsmechanik.” 
Finally, in a fifth chapter the author discusses the general problem to 
the solution of which he has set himself, and in a manner which disarms 
criticism, acknowledges that he has only taken a step or two towards 
its solution. That these steps are careful and scholarly and calculated 
to stimulate inquiry into one of the most difficult of problems, no one 
can question. 
Mechanical Genesis of Scales of Fishes.f — Mr. J. A. Ryder in 
making an attempt to give a mechanical explanation of the genesis of 
the scales of Fishes comes to two conclusions, which he thinks of import- 
ance. He finds that these scales bear a segmental relation to the other 
hard and to the soft parts, and that they are either repeated consecu- 
tively, and in oblique rows corresponding to the segments, or they may 
be repeated in rows as multiples of the somites ; there may, however, 
be segmental reduction which will affect the arrangement of the scales 
and reduce the number of rows. In the second place, the peculiar 
mode of interdigitation of the muscular somites, as indicated by the 
sigmoid outline of the myocommata, has developed a mode of insertion 
which has thrown the integument into rhombic areolae during muscular 
contraction. These areolae are in line in three directions, and the folds 
separating them are inflected in such a manner by muscular tension as 
to induce the condition of imbrication which is so characteristic of the 
squamation of many fishes. 
* Jenaische Zeitsclir. f. Naturwiss., xxvi. (1892) pp. 297-468 (14 pis.), 
f Proc. Acad. Philadelphia, 1892, pp. 219-24 (3 figs.). 
