i 
D.—-ZOOLOGY. 117 
groups or segments, one of which gives rise to the corona so typical of 
Serpulids. In the meanwhile, the posterior part of the body is reconstituted 
into thorax and abdomen. 
It is most desirable that these results should be fully tested on fresh 
material, but taken in conjunction with the work of Allen on Procerastea 
(10) and of the many workers on Lumbriculus, they point to the special 
nature of the apical segments of the body. Allen has shown that each 
batch of four or five segments taken from different portions of this Poly- 
chaet reorganise the whole, in such a manner, that the initial segments 
occupy the same relative position in the regenerated worm that they did 
in the parent animal; and Ivanov has shown in Lumbriculus, that the 
histological development of the seven anterior ‘ head’ segments follows 
a different course from that of the rest. Child has concluded from his 
studies on Planarians ‘ that the head which appears in the reconstitution 
of a piece is not physiologically part of the piece and is not formed by 
the piece, but develops, so to speak, in spite of it.’ (2. p. 113.) This is a 
hard saying, but we may bear it, if the facts I have given as to the process 
of head-formation in Polychaets are borne in mind. They show that the 
metabolic and morphological changes evoked by section are not those 
characteristic of the neck region in which they arise. They pursue a 
course of their own analogous to that followed by the normal pre-oral or 
apical lobe, and produce a complex structure in which the brain appears 
as a new development; whilst further back the new differentiator leads 
to the independent new formation of the mesoderm of the thorax and 
abdomen. The whole process is strikingly reminiscent of the two similar 
lines of metabolic activity in the embryo of the worm or of the frog, and 
constitutes confirmatory evidence of the existence of a co-ordinated system 
of gradients. 
A peculiar corollary arises out of a consideration of animals that may 
possibly present two apical regions at opposite ends of the major axis. I 
venture to suggest that Lamellibranchs might prove unusually interest- 
ing if examined from this point of view. It is also possible that Cestodes 
would give interesting results, especially in the case of those irregular 
growths known as Sparganum. One of the virtues of this hypothesis is 
that it makes old things new and suggests new problems for investigation. 
_ Above all, it has led to the power to predict and control the results of 
experiment on two groups of animals, the Oligochaetes and the Planarians. 
Summing up the evidence adduced in support of the ‘ gradient hypo- 
thesis,’ I am inclined to regard its value as indicative rather than demon- 
strative of that hypothesis, as its suggestiveness exceeds, in my opinion, 
its conclusiveness. Above all, this hypothesis suggests, and suggests per- 
haps for the first time, a method by which the problems of development 
can be linked up with those of genetics. 
Periodicity as a Fundamental Mode of Action. 
_ The animal according to this view is a system of periodic change. The 
‘system, as a whole, tends to slow down, but each part of it, each organ, 
works in shifts which permit every working group its period of rest. While 
resting, their capacity for output is increased, and on working again their 
tate of metabolism rises, falling again as the function progresses. Cycles 
of activity and morphological cycles are essentially age cycles. In the 
