188o. | A Sketch of Comparative Embryology. 871 
A SKETCH OF COMPARATIVE EMBRYOLOGY. 
BY CHARLES SEDGWICK MINOT. 
V.—THE GENERAL PRINCIPLES OF DEVELOPMENT. 
HE sponges present, as we have seen, many exceptional pecu- 
liarities in their development. All the remaining Metazoa, on 
the other hand, may be treated as members of one series, which 
are governed by several general laws of embryonic growth, only 
a portion of which can, at present, be said to apply to the 
sponges. 
The fundamental law of embryology is, that the simple pre- 
cedes the complex, the general and typical, the special. All em- 
bryos obey this principle in their early growth, and most of 
them throughout all their growth; but some, after advancing 
to a certain stage, stop, or suffer a degeneration as it is tech- 
nically called—in other words, only a part of their organs con- 
tinue to develop; or even the whole animal retrogrades, 2. e., 
becomes simpler. Of degeneration,’ the Crustacea offer many 
instances—one of the most familiar is the common barnacle, 
which in its young or larval state swims about the ocean freely, — 
having well developed limbs and sense organs, but later loses 
some of its structures, becoming in its adult condition perma- 
nently attached to the rock, Almost all parasitic forms are 
degraded. In spite of these instances, progress is primary and 
universal, degeneration secondary and exceptional. In all cases 
the embryos present to us animals stripped of the secondary - 
modifications found in adult life, and exhibiting the more essen- 
tial peculiarities, Thus in very young birds we plainly recognize, 
the gill slits and arches corresponding to the gills of fishes, but 
in the adult bird the gill slits have disappeared, and the arches so 
metamorphosed, that without knowing the embryo it would hardly 
be possible to discover their real connections, and their identity 
with the corresponding structures of fish. Embryology has 
proved that gills are typical of vertebrates, although many verte- 
brates have none in the adult state. Such insight the student of 
embryology may gather from any animal and every organ. 
The next law is, that development is always gradual—to it 
there are no exceptions. Even the sudden metamorphoses, e. g., 
1E. Ray Lankester has recently published a very interesting little volume on 
degeneration in the Nature Series. 
