PROBLEMS OF MODERN SCIENCE 



individual organism from the unicellular egg, 

 has occupied a large share of the attention of 

 biologists during the past century. To it we owe 

 the great generalisation known as the Recapitula- 

 tion Hypothesis, which has thrown a flood of light 

 upon the problems of organic evolution and done 

 much to facilitate a rational classification of plants 

 and animals. The classical case is that of the 

 Ascidians or sea-squirts. These highly degenerate 

 animals used to be regarded as invertebrates and 

 placed near the shellfish or Mollusca, until the 

 brilliant investigations of the Russian embryologist 

 Kowalesky showed that up to a certain point 

 their development follows upon typically chordate 

 lines, marked by the appearance of notochord, 

 central nervous system, and gill-slits exactly as in 

 higher vertebrates. In short, they pass through 

 an active chordate stage in their life-history, after 

 which they settle ^down to an inactive and sedentary 

 life and lose almost all traces of their aristocratic 

 ancestry. 



Embryology has long been regarded as the 

 final court of appeal in the interpretation of 

 anatomical structure, and though the celebrated 

 germ-layer theory has fallen somewhat into dis- 

 repute of late, there can be little doubt that the 

 exceptional modes of origin of certain organs in 

 particular cases, anomalous as they may seem, do 

 little more than emphasise the general applicability 

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