THE RISE OF EMBRYOLOGY 75 



observation and of technique. Studies have become 

 more intensive such as following the cell-succession 

 from the egg step by step until the germ-layers are 

 established, further investigations into the mech- 

 anism of development, the nature of fertilization 

 and the study of the behavior of the chromosomes of 

 egg and sperm. The subject has also become ex- 

 perimental. 



It has become too vast for a single publication to 

 embrace the embryology of invertebrates and verte- 

 brates, and, as a consequence, we have the standard 

 embryology of invertebrates by Korschelt and Heider 

 and many volumes on vertebrate embryology. There 

 are also those on a single animal, as Lillie's Develop- 

 ment of the Chick, and those on the development of the 

 Human Body such as the volumes by McMurrich, 

 Keibel and Mall, etc. 



The great reference book on vertebrate embryology 

 as a whole is the comparative and experimental 

 embryology of Vertebrates, in six volumes, under the 

 editorship of Oskar Hertwig, assisted by a large 

 number of collaborators. 



Among living and recent embryologists should be 

 mentioned: Oskar Hertwig (b. 1849) f Berlin; Wil- 

 helm His (1831-1904) of Leipsic, whose researches on 



