36 ADAM SEDGWICK. 
this subject. They have not as arule been clearly distinguished, 
and facts based on a study of larve have sometimes been 
assumed to hold true for embryos without further examination; 
and this practice has, as I hope to show, given rise to errors 
which have prevented our arriving at a clear understanding of 
the subject. 
It is, of course, impossible to examine fully all the stages of 
all Vertebrate embryos. In the comparison necessary for the 
criticism I am making it will be convenient to limit ourselves 
to typical cases, and I propose to consider (1) the embryos of 
animals widely divergent ; (2) the embryos of animals which 
are closely allied in the adult state. If it is found that in 
neither of these cases is the law of v. Baer followed, then I 
think we may reason that there is, to say the least of it, a 
strong probability that it will be found not to hold true for 
intermediate cases. 
(1) Embryos of divergent classes of the Craniate phylum. 
The examples I have chosen are the fowl and dog-fish. 
The fowl and the dog-fish in the adult state live under 
entirely different conditions ; whereas in the embryonic phases 
the conditions are very similar, both being developed within 
an egg-shell at the expense of ovarian yolk and surrounding 
albumen. 
According to the law of v. Baer these embryos ought to be 
closely similar in the young stage. 
Do these embryos, developing under similar conditions, con- 
form to the law? Superficially, clearly not. There is no 
stage of development in which the unaided eye would fail to 
distinguish between them with ease—the green yolk of the 
one, the yellow yolk of the other; the embryonic rim and 
blastopore of the fish, the absence of these in the chick ; the 
six large gill-slits bearing gills on the one hand, the four 
rudimentary clefts on the other; the small head, straight 
body, and long tail, as opposed to the enormous head, cerebral 
curvature, short tail, and so on. A blind man could distin- 
guish between them.' These embryos are not closely similar, 
1 J do not feel called upon to characterise the accuracy of the drawings of 
