THE GROWTH AND SCOPE OF BIOLOGY 



15 



a century and a half in which Uttle change took place in these instruments, 

 led to the discovery of the universal occurrence of cells. The credit for 

 this discovery belongs to no one person. Hooke had seen the boxlike 

 cavities in cork in 1665, and Malpighi observed those of other plant tissues 

 in 1670. Lamarck and Mirbel taught, early in the nineteenth century, 

 that plants and animals are composed of "cellular tissue." The nucleus 

 was sporadically seen and in 1833 recognized by Brown as of regular 

 occurrence in plants. His observation was verified by Schleiden, and 

 Schwann (Fig. 10) extended it to animals. The universal occurrence of 

 cells in living things was recognized by Dutrochet and Purkinje (Fig. 11), 



Fig. 10. Fig. 11. 



Fig. 10. — Theodor Schwann, 1810-1882. Fig. 11. — Johannes Evangelista Purkinje, 



1787-1869. {Both from Garrison, " History of Medicine.") 



and a formal statement of that universality was published by Schwann in 

 1839. Knowledge of the nature of cells was gradually accumulated 

 through the work of various biologists, culminating in the convincing 

 proof by Max Schultze, about 1861, that the essential feature of living 

 things is the jellylike substance called protoplasm, which was at first 

 regarded as merely incidental. 



This knowledge of cells had a profound influence upon further 

 advances in morphological biology. The study of tissues, begun several 

 decades before, now became a study of like cells grouped together. 

 Embryology was pushed back to the very beginning of development, 

 to the egg cell, and the so-called germ layers (of cells) in the embryo of 

 the chick were discovered. Unfortunately, knowledge of the minute 

 structure of cells was not sufficient until much later to influence physiolog- 



