?4 BIOLOGY AND ITS MAKERS 



the cells were not thought of as uniform elements of organic 

 architecture, and no theory resulted. It is true that Malpighi 

 understood that the cells were separable " utricles," and that 

 plant tissue was the result of their union, but this was only 

 an initial step in the direction of the cell-theory, which, as 

 we shall see later, was founded on the supposed identity in 

 development of cells in animals and plants. Fig. 73 shows 

 a sketch, made by Malpighi about 1670, illustrating the micro- 

 scopic structure of a plant. This is similar to the many 

 drawings of Grew and Leeuwenhoek illustrating the struc- 

 ture of plant tissues. 



Wolff. Nearly a century after the work of Malpighi, we 

 find Wolff, in 1759, proposing a theory regarding the organ- 

 ization of animals and plants based upon observations of 

 their mode of development. He was one of the most acute 

 scientific observers of the period, and it is to be noted that his 

 conclusions regarding structure were all founded upon what he 

 was able to see; while he gives some theoretical conclusions 

 of a purely speculative nature, Wolff was careful to keep 

 these separate from his observations. The purpose of his 

 investigations was to show that there was no pre-formation 

 in the embryo; but in getting at the basis of this question, he 

 worked out the identity of structure of plants and animals 

 as shown by their development. In his famous publication 

 on the Theory of Development ( Theoria Generationis) he used 

 both plants and animals. 



Huxley epitomizes Wolff's views on the development of 

 elementary parts as follows: "Every organ, he says, is com- 

 posed at first of a little mass of clear, viscous, nutritive fluid, 

 which possesses no organization of any kind, but is at most 

 composed of globules. In this semifluid mass cavities 

 (Bldschen, Zellen) are now developed ; these, if they remain 

 round or polygonal, become the subsequent cells; if they 

 elongate, the vessels ; and the process is identically the same, 



