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brought into play to determine the growth of a tissue from an 

 amorphous blastema, and, moreover, to give the elements of 

 that tissue peculiar and characteristic properties ; and, lastly, 

 we have to inquire how various tissues combine to form organs. 

 Each of these problems requires for its solution a greater 

 amount of chemical and physico-physiological knowledge than 

 we yet possess. 



The researches of Turpin and Dumortier, Schleiden, 

 Schwann, Henle, Valentin, Reichert, and Others, have thrown 

 considerable light on the manner of growth of tissues, and no 

 reasonable doubt can be now entertained that the Cell-theory, 

 as elaborated by these and other observers, and specially ap- 

 plied to the development of animal structures by Schwann, 

 fully and satisfactorily explains the mode of formation and de- 

 velopment of several tissues and organs. Notwithstanding, 

 however, the reception of this theory, even in its extreme and 

 exclusive application , as insisted on by Schwann, in mos t schools, 

 soon after its promulgation, it has been found defective in many 

 points, and has been and is still questioned by several observers 

 of authority. Having devoted much attention to this subject, 

 I have become convinced by repeated observations that there 

 are several tissues which at no period of their development 

 exhibit any evidence of formation by cells, and consequently 

 that cells cannot be considered as the only plastic germs or 

 formative elements of organic life. 



The following researches and observations appear to me to 

 support the opinion now stated. 



As the common hen's egg offers great facilities for the study 

 of the formation of structures, I have made some careful ob- 

 servations of the microscopic elements, which it presents both 

 before incubation and at certain periods after the commence- 

 ment of that process. 



1 . A portion taken from the mass of the yolk, and sub- 

 mitted to a power of 420 D., exhibited the following elements : 



a. An abundance of minute granules, covering the greater 

 part of the field, of a light-yellowish colour, with a dark border 



