Septembee 7, 1900.] 



SCIENCE. 



363 



credence, especially amongst those who were 

 engaged in the study of pathological proc- 

 esses, that the origin of cells within pre- 

 existing 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 be no longer needed. 

 Without doubt tliis 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 pre-existing parents, in or- 

 ganic 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 experi- 

 mentally refuted, on the other hand it has 

 not been experimentally proved. The bur- 

 den 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 di- 

 vision, 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 pub- 

 lished 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 observation 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 pos- 

 sessed a nucleus. The vesicles were obvi- 

 ously cells which had arisen within the 

 original germ-cell or ovum. These changes 

 were systematically described by Martin 

 Barry so long ago as 1839 and 1840 in two 

 memoirs communicated to the Koyal So- 

 ciety of London, and the appearance pro- 

 duced, on account of the irregularities of 

 the surface occasioned by the production of 

 new vesicles, was named by him the mul- 

 berry-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 foun- 

 dations of other cells. He recognized that 

 the new cells were derived from the ger- 

 minal vesicle or nucleus of the ovum, the 

 contents of which entered into the for- 

 mation 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 pre-existing cell and the 

 process which we now term the segmenta- 

 tion of the yolk were successfully demon- 

 strated. In a third memoir, published in 

 1841, Barry definitely stated that young 

 cells originated through division of the 

 nucleus of the parent cell, instead of arising, 

 as a product of crystallization, in the fluid 

 cytoblastema of the parent cell or in a blas- 

 tema situated external to the cell. 



In a memoir published in 1842, John 

 Goodsir advocated the view that the nu- 

 cleus is the reproductive organ of the cell, 

 and that from it, as from a germinal spot, 

 new cells were formed. In a paper, pub- 

 lished three years later, on nutritive cen- 

 ters, he described cells, the nuclei of which 

 were the permanent source of successive 

 broods of young cells, which from time to 

 time occupied the cavity of the parent cell. 

 He extended also his observations on the 



