?6 ADAM SEDGWIOK. 



this subject. They have not as a rule been clearly distinguished, 

 and facts based on a study of larvae 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, 

 • I do not feel called upon to characterise the accuracy of the drawings of 



