LAW OP DEVELOPMENT KNOWN AS VON BAEU'S LAW. 83 



aflFected 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 shortj 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 Elasmobrancbii 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. xxxiii, 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 Teleosteaus 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., pi. xxxvii, fig. 43), but, so far as I am aware, his 

 interpretation of this structure has not been confirmed. InAmniota — 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 sufiiciently 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 Ichtbyopsida, 

 while it is present in relatively normal development in all Amniota. 



