

ABBKEVIATION OF ONTOGENY 



'273 



du 



the characteristic piscine organs are fully functional, but even in 

 the embryos of higher air-breathing vertebrates reptiles, birds 

 and mammals while enclosed within the egg-shell or within the 

 womb of the parent, we can still recognize clear traces of a 

 similar fish-like stage in their evolution, for gill-slits are present 

 in all (Fig. 122, g.s.) and the 

 arrangement of the principal veins 

 and arteries is unmistakably fish- 

 like. 



Difficulties in the interpretation cf 

 the ontogenetic record arise not only 

 from the addition of new features 

 in the form of embryonic or larval 

 organs but also from the abbreviation 

 of the different stages through which 

 the organism passes. The develop- 

 ment of any of the higher organisms 

 must be looked upon as a kind of 

 compromise, due to the necessity of 

 attaining as perfect a condition as 

 possible in as short a time as pos- 

 sible, for the sooner the adult condi- 

 tion is reached the sooner will the 

 organism be able to reproduce its 

 kind, and hence any abbreviation of 

 the earlier stages of development will 

 be advantageous to the species and 

 will be favoured by natural selection. 

 For this reason the fish-like stage 

 in the development of reptiles, birds 

 and mammals is passed through very 

 rapidly and the fish-like structure is 

 no longer fully developed. Although gill-slits are present the 

 gills themselves are not, and other piscine characters are only 

 represented by transient vestiges. 



Turning now to some of the lower groups of the animal king- 

 dom, we may briefly notice a beautiful illustration of the law of 

 recapitulation afforded by the life-history of the common feather- 

 star, Antedon (Fig. 123). This animal, found abundantly in 

 comparatively sjiallow water off the British coast, belongs to the 

 great phylum Echinodermata, which also includes the star-fish, 



FIG. 122. Chick Embryo of 

 about 3 Days, with the 

 Foetal Membranes and 

 Yolk -Sac removed. (Pho- 

 tographed from one of 

 Ziegler's models.) 



au., auditory organ ; e., eye ; g.s., gill 

 slits; n., heart; m.s., meso- 

 blastic somites ; n., nasal organ. 



