38 The Recapitulation Theory and Human Infancy 



segmented and coelomate condition. The cavities of the coelo- 

 mic pouches form the coelom or body cavity, which is at first 

 transversely sub-divided into compartments, as it still is in the 

 adult earthworm, while their walls form the third germ-layer 

 or meso-blast, lying between the epiblast which covers the sur- 

 face of the body and the hypoblast which lines the digestive 

 cavity. Along the mid-dorsal line of the body a strip of epi- 

 blast sinks down and becomes folded into the form of a tube, the 

 rudiment of the central nervous system (brain and spinal cord) 

 and beneath this tube a long strip of hypoblast becomes nipped 

 off ffom the roof of the gut, forming the notocord or axial skele- 

 tal rod — the foundation around which in higher types the verte- 

 bral column is built up. A little later the front part of the gut 

 becomes pierced by gill-slits for purposes of respiration and the 

 primitive chordate condition is thus fully attained. 



Amphioxus does not progress much beyond this stage in its 

 development. . . .Amphioxus is a chordate animal but it is not a 

 true vertebrate. It probably, however, represents fairly close- 

 ly a stage of evolution through which the ancestors of the verte- 

 brates have passed, though it becomes somewhat modified by 

 secondary features in the later stages of its development." 62 



In the case of man Haeckel carried the series beyond that 

 represented by Amphioxus through three stages, the cyclostoma, 

 the ichthyoda, and the amniote, making ten stages in all re- 

 presenting an ontogenetic-phylogenetic correspondence. 63 But 

 as suggested above in the account of Haeckel's part in the for- 

 mulation of the theory of recapitulation, the ancestral refer- 

 ence in the higher stages is often to ontogenetic conditions in 

 lower forms, making the adult phylogenetic equivalence in such 

 stages indirect, to say the least. 



It is significant, by the way, that the chief exponent of recap- 

 itulation is able to present an ontogenetic-phylogenetic cor- 

 respondence of ten stages only. When it is considered that on 

 the phylogenetic side the sequence occupied millions of years 

 and proceeded through an almost indefinite number of special 

 forms, the limitations of a conception of ontogeny as illus- 

 trative of the chronological sequence of ancestors become fairly 

 obvious. 



It is almost impossible to report biological opinion as to the 

 significance of the kind of fact now referred to, so various are 

 the views held regarding it. One can merely set the outside- 



'» Outlines of Evolutionary Biology, 1912, pp. 265-267. 

 " Evolution of Man (5th ed. trans.), pp. 547-550. 



