50 
IOWA ACADEMY OF SCIENCES. 
Nervous tissue is rapidly developing an extensive and 
highly ramified literature of its own. The tendency is 
current, on the part of at least a few neurologists, to 
neglect the boundaries of the nerve-cell altogether, plac- 
ing emphasis, instead, on the continuity of neuro-fibrils 
throughout the entire nervous system. It may safely be 
said, however, that a healthy condition of the subject will 
require proper recognition of cell-values. 
VI. THE REPRODUCTION OF THE CELL. 
The phenomena of development are recorded in a litera- 
ture which has become truly voluminous, but only during 
very recent years has the work passed beyond the purely 
descriptive stage. Experimental embryology has made 
very rapid progress, however, and the achievements 
already to- its credit are so noteworthy as to give promise 
of a brilliant future. 
Fertilization is now definable as a distinctly twofold 
process. The factor first recognized, the transfer of chro- 
matin, has long been known in the most minute detail. 
The other factor, but recently distinguished as such, is the 
stimulus to development which results from the entrance 
of the spermatozoon. Oogenesis leaves the ovum in what 
may be called the first critical stage of the organism, in 
which the living material has reached a condition of such 
stability that death soon occurs unless the stimulus of fer- 
tilization be given. This stimulus is conveyed by the 
spermatozoon normally; but the experimental studies of 
Loeb and others have made it clear that a variety of stim- 
uli may be productive of artificial parthenogenesis. A 
change of osmotic pressure, even mechanical shock, will 
induce development from this critical condition. Eggs 
which are normally parthenogenetic find the stimulus 
proper to their development amid natural surroundings, 
without the intervention of fertilization at all. 
A few lines of work are now to be touched very lightly, 
and the bare mention of others must be omitted altogether. 
In the field of mitosis, the models of Heidenhain have at 
