THE HISTORY OF ANIMAL LIFE 411 



the rabbit the pouch at the junction of the small and large intestine, 

 the caecum, is nearly as long as the large intestine itself. In higher 

 fishes there are a number of small pouches (intestinal caeca) around 

 the small intestine; in the lower fishes, for example the shark, the 

 vestige of the typhlosole of the Annelida is found modified as a 

 spiral valve. One may regard the digestive system of a member of 

 any vertebrate group as representing an alteration of a basic plan 

 common to all vertebrates, the basic plan being in itself evidence 

 of a common ancestry of vertebrates. 



Evidence from Embryology. It is apparent to any observer 

 that the developing embryo of animals high in the scale passes 

 through stages and develops structures that are reminiscent of lower 

 forms. Many years ago Miiller expressed this as a biogenetic law, 

 namely, that the development of the individual repeats the history 

 of the race; Haeckel vigorously propounded and enlarged the doc- 

 trine, sometimes expressed by the high sounding phrase that "The 

 Ontogeny repeats the Phylogeny." The limitations of this concept 

 have been discussed elsewhere; the conclusion was reached that 

 at no time is an embryo of one of the higher animals identical with 

 the adult of a lower form and that the law of biogenesis must be 

 restricted to mean that in a general way the processes of develop- 

 ment of higher animals repeat the processes of development of ani- 

 mals lower in the scale. The nature of some of the evidence for 

 this law is indicated in the paragraphs which follow. 



From time to time in previous chapters various facts of devel- 

 opment have been mentioned and discussed as historical in their 

 significance. It is necessary here only to list some of the high lights 

 that illumine the origin of the peculiar structures that appear in 

 development: 



(i) An early stage of development of all Metazoa consists of a 

 single-layered hollow sphere of cells or some modification of such 

 a structure, termed the blastula. Its geometrical shape is reminiscent 



