MODERN BIOLOGY 5^9 



these investigations ultimately became somewhat monotonous, with the re- 

 sult that this line of research finally became quite unmodern and the interest 

 began to turn in other directions. Among the investigators who compiled 

 the results of this work may be mentioned Robert Wiedersheim (1848-1913), 

 a disciple of Leydig and professor at Freiburg, well known for his compre- 

 hensive work on the anatomy of the Vertebrata as well as his studies of 

 special subjects, particularly of the bone-structure of the Batrachia, and the 

 Swiss, Arnold Lang (1855-1916), a disciple of Haeckel and professor at 

 Zurich, who wrote a widely referred-to work on the anatomy of the in- 

 vertebrates and a number of monographs on various groups among the 

 worms, 



Etnbryology 

 That branch of morphology, however, that was specially developed under 

 the influence of the descent theory was embryology. The biogenetical prin- 

 ciple and its related subjects, the theories of germinal layers and the gastrasa, 

 were applied to different spheres and gave rise to ideas in many directions, 

 besides which the new microtechnics offered a means for detailed discover- 

 ies of hitherto undreamt-of results. Embryology, therefore, proves to have 

 been the most productive of the morphogenetical specialized spheres and 

 attracted to it the most eminent biologists of the time. 



Among these representatives of phylogenetical embryology only a few 

 of the more important can be mentioned here. Alexander Kowalewsky 

 (1844-1901), an academician of St. Petersburg, worked in the spirit of 

 Haeckel, encouraged by his commendation; his detailed investigations into 

 the development of ascidians and salpa; covered an immense amount of de- 

 tail and the same is true of his work on the development of the lancet-fish, 

 with the result that even the ontogeny of this much-discussed animal be- 

 came known. Kowalewsky was a firm supporter of the theory of the ger- 

 minal layers and developed it by making contributions of his own in the 

 theoretical sphere. 



The same line of research was also followed by the two brothers Hert- 

 wig, and it led them both to make discoveries of fundamental importance 

 and to produce theoretical ideas of a very different nature from those from 

 which they had started. Oscar Hertwig was born in 1849 and Richard 

 Hertwig in 1850, the sons of a merchant at Friedberg in Hesse. They both 

 studied at Jena under Haeckel and became lecturers there and finally pro- 

 fessors, Oscar of anatomy at Berlin, Richard of zoology at Munich. Both 

 carried on, each in his own subject, extensive and important activities as 

 teachers and investigators. At Jena they worked together in the sphere of 

 evolution in the manner of Haeckel and published a series of papers entitled 

 Studien x.ur Blattertbeone, which dealt especially with the problem of the mid- 

 dle germinal layers. Here they expounded their famous "coelom" theory, 



