530 THE HISTORY OF BIOLOGY 



which was intended to be a universal answer to the question: "How does 

 the two-layered embryo develop into a higher organization?" The theory 

 takes as its starting-point the two primary germinal layers, the ectoderm 

 and the entoderm, between which there arises at an early stage an originally 

 structureless layer formed by immigrating cells, which is here termed "mes- 

 enchyme." The animals are now divided, in respect of their development, 

 into two groups, dependent upon whether the mesenchyme participates in 

 the formation of tissue or not. The former takes place chiefly in the coral 

 animals, the flat-worms and the molluscs, in which the muscular and nerv- 

 ous systems are formed out of the mesenchyme, whereas in most other ani- 

 mal types, chiefly the Articulata and the Vertebrata, the said tissues are of 

 purely epithelial origin and are formed out of a dual evagination of the 

 entoderm, the inner cavity of which gives rise to the body cavity, or the 

 coelom. The theory was afterwards applied, after a series of special investi- 

 gations, to the organic formation of different animal forms and won general 

 acceptance at the time. It is true, His declined to accept it, but did not suc- 

 ceed in substituting any better explanation. Later research, however, has 

 found this theory to be far too schematical; students have given up referring 

 the various organs to the three germinal layers and now instead seek their 

 origin, each separately, in so-called primitive rudiments. Furthermore, the 

 formation of the coelom through simple invagination has been found upon 

 closer investigation to be far less frequent than the two brothers imagined. 

 Their theory has nevertheless played its important part and has called forth 

 abundant special research-work of value for all time. In the following pages 

 we shall repeatedly find their names mentioned in connexion with valuable 

 contributions to the advancement of biology. Among their pupils may be 

 cited the scientific collaborators Eugen Korschelt (born in 1858, professor 

 at Marburg) and Karl Heider (born in 1856, latterly professor at Berlin), 

 who together published an exhaustive summary of the knowledge of their 

 time regarding the evolution of the invertebrates. Moreover, both have dis- 

 tinguished themselves as specialists, particularly in the sphere of experimen- 

 tal research. 



During this period England was also the scene of valuable embryologi- 

 cal research-work. Among her representatives may be mentioned Edwin 

 Ray Lankester (born 1847), a professor at the British Museum and author 

 of a number of papers on evolution, dealing especially with the fishes and 

 the Articulata. He especially took up for study and further elaborated the 

 coelom theory and has brought it to the highest point it has yet reached, 

 having sought to base on it the classification of the animal kingdom. A 

 very distinguished name in the sphere of evolution has been won by Francis 

 Maitland Balfour, who was born in 1851 and died, as the result of an 

 accident, in i88i, the younger brother of the famous statesman Lord Balfour. 



