LAW OF DEVELOPMENT KNOWN AS VON BAER’S LAW. 43 
affected the whole of embryonic development. As examples 
I may mention the small outer toes on the feet of the pig 
and probably of other Ungulates, the large digit of the ostrich’s 
foot and of the kangaroo’s foot, the spiracle of Elasmobranchii,! 
the rudimentary character of the phalanges of the bird’s hand. 
I have no doubt that many other instances will occur to my 
reader. (3) Organs which have been recently acquired may 
appear at the very earliest possible stage; e.g. the double 
hallux present in some breeds of fowls makes its appearance 
as soon as the other digits; the webbing of the duck’s foot 
is not preceded by a stage in which the digits are separate. 
In short, the evidence seems to indicate that in a great number 
of cases adult variations of any part are accompanied by 
precedent similar alteration of the same part in the embryo. 
I do not mean to affirm that the alteration of the organ in 
' The spiracle of Elasmobranchii is a reduced gill-cleft, and in correspond- 
ence with its reduction in the adult it is found to be reduced also in all 
stages of its development from its very first appearance, which takes place 
after the hyobranchial cleft—not before it, as would be expected from its 
position as the anterior member of a series (see Self, “‘ Notes on Elasmobranch 
Development,” this Journal, vol. xxxili, p. 572). It would be excessively 
interesting in this connection to ascertain whether any trace of the spiracle 
is present in the embryos of those Elasmobranchs in which it is absent in the 
adult. In fact, an account of the spiracular cleft throughout the Vertebrata 
is much needed. Is it present in embryo in Teleosteans and in Lepidosteus ? 
Balfour asserts that it is present in the former (‘Comp. Embryology,’ vol. ii, 
p. 77, mem. ed.), but I am unable to find his authority for the statement. 
He also states that it is present in the embryo Lepidosteus as a double layer 
of cells without a cavity (Balfour and Parker on ‘ Anatomy and Development 
of Lepidosteus,’ mem. ed., pl. xxxvii, fig. 43), but, so far as I am aware, his 
interpretation of this structure has not been confirmed. In Amniota—through- 
out which the pharyngeal clefts present a very remarkable constancy—the 
spiracular cleft is as large, if not larger, than the succeeding one, and appears 
first in development. ‘This is an exceedingly interesting fact, which has not 
been sufficiently noted. It tends to show that the Amniota have arisen from 
aquatic forms independently of the terrestrial Amphibia, in which group the 
spiracular cleft is not formed at all—though a slight rudiment of it does 
appear for a short period. In fact, we may take it as a fact of systematic 
value that the spiracular cleft is absent or rudimentary in all Ichthyopsida 
while it is present in relatively normal development in all Amniota, 
