The Recapitulation Theory in Biology 37 



a law of recapitulation, but at most only the retention of ances- 

 tral forms. Whether these are to be regarded as embryonic 

 or adult forms is a matter for study in each case. 



(c) The Facts of Comparative Morphology. 



The third type of evidence for recapitulation, that involving 

 a comparison of ontogenetic stages with forms common to "low- 

 er" existing groups, may now be referred to. There are, first, 

 those phases in the very early embryonic development of Met- 

 azoa suggestive of a progression from the unicellular condition, 

 and in the development of Chordata, from the simple metazoon 

 form to the chordate and vertebrate condition. The case for 

 recapitulation on these lines is well stated by Dendy in his 

 recent text-book. 



"'We have already referred. . . .to some of the earlier stages 

 in the development of that primitive fish-like animal Amphioxus. 

 Let us next enquire how these stages may be interpreted in accor- 

 dance with the recapitulation hypothesis. The unicellular 

 ovum obviously represents the remote protozoon ancestors which 

 were common to the whole animal kingdom, and which are also 

 represented at the present day by independent unicellular or- 

 ganisms such as the Amoeba. The segmentation of the ovum 

 into primitive embryonic cells or blastomeres represents the 

 transition from the condition of the simple protozoon to that of 

 the protozoon colony, in which the individual cells, instead of 

 separating, as in the dividing Amoeba, remain together, but still 

 without undergoing any marked differentiation and division of 

 labor. The arrangement of the blastomeres in the form of a 

 hollow sphere, the blastula or blastosphere, with a single layer 

 of cells surrounding a central cavity, represents the formation 

 of such a protozoon colony as we see in the existing Volvox or 

 Sphaerozoum. The process of gastrulation, whereby the single- 

 layered blastula is converted into a two-layered gastrula, with 

 primitive digestive cavity (enteron) and primitive mouth (blas- 

 topore) represents the transition from the protozoon colony to 

 the coelenterate stage of evolution, the latter being still repre- 

 sented at the present day by such forms as Hydra, Obelia, the 

 jelly-fish, the corals, and all their numerous relations, which 

 retain in their organization all the essential features of the gastru- 

 la, though generally complicated by the development of tentacles, 

 skeleton, etc. The development of the coelomic pouches as 

 outgrowths of the primitive digestive cavity, and the conversion 

 of these into mesoblastic somites, arranged serially or metameri- 

 cally down each side of the body, mark the transition from the 

 unsegmented and coelenterate condition to the metamerically 



