44 H. J. Clark on the Microscope. 
micrometer screw, which works the fine adjustment of the ob- 
jective; every cell, se wer may be treated as if it were a sepa- 
rate out W would warrant to measure, for instance, the 
size of the sins of | a nerve after it had been removed from its 
natural position, and with more or less inevitable distortion? 
Unfortunately, investigators have been compelled to do this too 
often, up to this very ay but now I as for much better and 
the eis dorsalis, intestines, vertebree, muscles, Xc., similar 
and apparently gradual changes have been observed ; but each 
step, in most instances, was investigated isolately from the pre- 
vious one, and the intervening pas bridged over by the process 
of inductive reasoning alon This is not enough; now we 
know that every sec péond of thei life of a cell, or series of cells, 
on be Danse most minutely, minute by minute, hour by hour, 
day by day. Day and night, watches have been kept by | 
lon Me in other departments of science, and why may not the 
naturalist do so? In some cases a eae extensive series of 
Wyman has observ m the segmenting of the yolk 
to hatching in the space of about forty hours.. It is not possible, 
in = way, to trace the grad «se vam of cells and 
I have in ‘mind a remarkable instance of the evils of the eee 
monomaniacal habit of using pressure whilst investigatin tis- 
sues. A celebrated physiologist, i in_all probability, m the | 
most fortunate chance of discovering the key to the whole 
history of the mode of origin of the embryo from the tara 
the segmentation of the y ytho full- — 
reptiles, and 
