DIVISIONS OF ZOOLOGY. 7 
over its surface in a saucer-like form. In this case there are: 
no morula nor planula stages, and the gastrula stage is formed 
by epiboly. 
The next step in development is the formation of a. 
third layer—the mesoblast—between the epiblast and the 
hypoblast. From these three layers, all the organs, which, 
taken collectively, form the individual, are derived. All the 
metazoa go through these stages, but afterwards, differences 
appear in the different groups, and as development goes on, 
the divergence between different forms becomes greater and 
sreater. This is the meaning of Von Baer’s law, that: 
“development is from the general to the special.” These 
organs generally develop symmetrically round an axis. 
When the parts correspond on two sides only, the animal is 
said to be bilaterally symmetrical; when the parts corres- 
pond on more than two sides, the animal is said to be 
radially symmetrical. In many cases the parts are repeated 
behind each other along the axis, thus dividing the animal 
into segments, or metameres. In this case, the animal, in 
addition to its bilateral or radial symmetry, is said also to 
possess serial symmetry. From these segments outgrowths 
may arise, called appendages, and these appendages may be 
similar or dissimilar. The appendages are moved by 
muscles attached to them. A muscle may be a continuous 
sheet of muscular fibres lying in one or many directions, 
and surrounding or lining an organ, or it may consist of a 
bundle of fibres free in the middle, and attached at each 
end. In this case the attachment nearest to the axis of the 
body is called the origin of the muscle; that furthest away 
trom the axis, its insertion. A muscle may be attached 
directly, or one or both ends may be produced into a tendon 
composed of white fibrous tissue. 
When, in compariag the development of two different 
