880 A Sketch of Comparative Embryology. |December, 
the skin of many animals there are unicellular glands. Every mi- 
nute scale making the microscopic 
dust on a butterfly’s wing results from 
the modification of a single cell, from 
which the scale grows out; again in 
VE VRA, the ectoderm of Ceelenterates, Fig. 28, 
oy i A } we usually find scattered among the 
o Ne P >Z unmodified epidermal cells single net- 
A tle cells (thread or lasso cells), Z, and 
unicellular glands. The nettle cells 
Fic. 27. — Transverse section may be readily recognized by the 
aa e PETE glands of a do- Coiled thread in each of them; the 
“ hauptzellen;” , peripheral gland cells by a small pore and their 
cells. After Rollet. Xx 320 diam. é = 
mass of secretion, Fig. 28 D. 
Such are the leading principles of embryology as far as our 
space permits dealing with them, varying 
however in their exact application from group 
to group. In all the embryos of each natural 
group, we can recognize peculiarities common 
to all the members of the group, peculiarities 
which we therefore designate as typical. 
When, however, the embryonic form leads a 
free life, it may often present special adapta- 
tions that change it so much as to obscure the 
ry typical features, hence in the study of those 
forms which begin their free life in an embry- 
onic condition, we have to compare the larve, 
pores le gland cell. Af- one with another, in order, by the elimination 
of those features which are only special and 
secondary, to discover the really typical structure. This is par- 
ticularly the case with marine animals, whose larvæ often have 
bizarre shapes, which have arisen, it is to be assumed, by natural 
selection among the larvæ, and relate to their presentation rather 
than directly to their development. Therefore we shall not pause 
to consider the forms of embryonic larvæ. I hope, however, to 
publish, before long, figures which will enable the student to 
recognize the more common marine embryos, 
