ADDRESS BEFORE THE BRITISH ASSOCIATION. 569 



amongst those who were engaged in the study of pathological processes, 

 that the origin of cells within preexisting cells was to a large extent 

 lost sight of. That a parent cell was requisite for the production of 

 new cells seemed to many investigators to he no longer needed. With- 

 out doubt this conception of free cell-formation contributed in no small 

 degree to the belief, entertained by various observers, that the simplest 

 plants and animals might arise, without preexisting parents, in organic 

 fluids destitute of life, by a process of spontaneous generation; a belief 

 which prevailed in many minds almost to the present day. If, as has 

 been stated, the doctrine of abiogenesis cannot be experimentally re- 

 futed, on the other hand it has not been experimentally proved. The 

 burden of proof lies with those who hold the doctrine, and the evidence 

 that we possess is all the other way. 



MULTIPLICATION OF CELLS. 



Although von Mohl, the botanist, seems to have been the first to 

 recognize (1835) in plants a multiplication of cells by division, it was 

 not until attention was given to the study of the egg in various animals 

 and to the changes which take place in it, attendant on fertilization, 

 that in the course of time a much more correct conception of the origin 

 of the nucleus and of the part which it plays in the formation of new 

 cells was obtained. Before Schwann had published his classical 

 memoir in 1839, von Baer and other observers had recognized within 

 the animal ovum the germinal vesicle, which obviously bore to the 

 ovum the relation of a nucleus to a cell. As the methods of observa- 

 tion improved, it was recognized that, within the developing egg, two 

 vesicles appeared where one only had previously existed, to be followed 

 by four vesicles, then eight, and so on in multiple progression until the 

 ovum contained a multitude of vesicles, each of which possessed a 

 nucleus. The vesicles were obviously cells which had arisen within the 

 original germ-cell or ovum. These changes were systematically de- 

 scribed by Martin Barry so long ago as 1839 and 1840 in two memoirs 

 communicated to the Royal Society of London, and the appearance 

 produced, on account of the irregularities of the surface occasioned by 

 the production of new vesicles, was named by him the mulberry-like 

 structure. He further pointed out that the vesicles arranged themselves 

 as a layer within the envelope of the egg or zona pellucida, and that the 

 whole embryo was composed of cells filled with the foundations of other 

 cells. He recognized that the new cells were derived from the germinal 

 vesicle or nucleus of the ovum, the contents of which entered into the 

 formation of the first two cells, each of which had its nucleus, which in 

 its turn resolved itself into other cells, and by a repetition of the process 

 into a greater number. The endogenous origin of new cells within a 

 preexisting cell and the process which we now term the segmentation 



