364 THE HISTORY AND SCOPE OF ZOOLOGY IX 



and Eemak, that all cells originate by fission from 

 pre-existing cells, — a fact unknown to Schwann, — 

 and the doctrine " omnis cellula e cellula " was estab- 

 lished. It Y\\as also demonstrated that the Mammalian 

 egg discovered by Yon Baer was a typical nucleated 

 cell, and that all animals, and plants also (this general- 

 isation took thirty years to establish), take their 

 origin from an egg, which is in essence and in fact a 

 single nucleated cell. The doctrine of Harvey, " omne 

 vivum ex ovo," thus received its most ample justifica- 

 tion. The study of '^ growth from the egg " became 

 necessarily a study of the multiplication by fission of 

 the egg-cell and its fission-products, their arrangement 

 in layers, and the chemical metamorphosis of their 

 substance and exudations. This study, as well as 

 the allied investio'ation of the cell-structure of the 

 adult tissues, was immensely facilitated by methods 

 of hardening, staining, section-cutting, and clarifying 

 which grew up after Schwann's time, and have their 

 present highest development in the automatic micro- 

 tome of Caldwell, which can be worked by a motor, 

 and delivers consecutive sections of animal tissues or 

 embryos ^Q^j^th of an inch thick, arranged in the 

 form of ribbons, ready for examination with the micro- 

 scope, at the rate of one hundred or more per minute. 

 Strieker of Vienna was the first to embed embryos in 

 waxy material for the purpose of cutting thin sections 

 of them, about twenty-five years ago, and E. Leuckart 

 of Leipsic was subsequently the first to employ this 

 method in the study of the structure of small In- 

 vertebrata. 



