D.—ZOOLOGY. 115 
The culture of tissues outside the body is throwing new light on the 
conditions requisite for the multiplication and differentiation of cells. 
R. G. Harrison (1907) was the first to devise a successful method by 
which the growth of somatic cells in culture could be followed under 
the microscope, and he was able to demonstrate the outgrowth of nerve- 
fibres from the central nervous tissue of the frog. Burrows (1911), 
after modifying the technique, cultivated nervous tissue, heart-cells, 
and mesenchymatous tissue of the chick in blood-plasma and embryonic 
extract, and this method has become a well-established means of 
investigation of cell-growth, tissues from the dog, cat, rat, guinea-pig, 
and man having been successfully grown. One strain of connective 
tissue-cells (fibroblasts) from the chick has been maintained in culture 
in vigorous condition for more than ten years, that is for probably some 
years longer than would have been the normal length of life of the cells 
in the fowl. Heart-cells may be grown generation after generation—all 
traces of the original fragment of tissue having disappeared—the cells 
forming a thin, rapidly growing, pulsating sheet. Drew (1922) has 
recently used instead of coagulated plasma a fluid medium containing 
calcium salts in a colloidal condition, and has obtained successful growth 
of various tissues from the mouse. He finds that epithelial cells when 
growing alone remain undifferentiated, but on the addition of connective 
tissue differentiation soon sets in, squamous epithelium producing 
keratin, mammary epithelium giving rise to acinous branching struc- 
tures, and when heart-cells grow in proximity to connective tissue they 
exhibit typical myofibrillz, but if the heart-cells grow apart from the 
connective tissue they form spindle-shaped cells without myofibrille. 
This study of the conditions which determine the growth and differentia- 
tion of cells is only at the beginning, but it is evident that a new line 
of investigation of great promise has been opened up which should lead 
also to a knowledge of the factors which determine slowing down of the 
division-rate and the cessation of division, and finally the complete 
decline of the cell. 
For many lines of work in modern zoology biochemical methods are 
obviously essential, and the applications of physics to biology are like- 
wise highly important—e.g. in studies of the form and development of 
organisms and of skeletal structures. Without entering into the vexed 
question as to whether all responses to stimuli are capable of explanation 
in terms of chemistry and physics, it is very evident that modern develop- 
ments have led to the increasing application of chemical and physical 
methods to biological investigation, and consequently to a closer union 
between biology, chemistry, and physics. It is clear also that the 
association of zoology with medicine is in more than one respect 
becoming progressively closer—comparative anatomy and embryology, 
cytology, neurology, genetics, entomology, and parasitology, all have 
their bearing on human welfare. 
Some Bearings of Zoology on Human Welfare. 
The bearings of zoology on human welfare—as illustrated by the 
relation of insects, protozoa and helminthes to the spread or causation 
of disease in man—have become increasingly evident in these later years 
