TRANSACTIONS OF SECTION D. 499 
greater than the average size of Saccammina, whereas, according to Rhiimbler’s 
theory, it should have passed into its adult stage; and finally it presents two 
well-marked varieties, var. parva (Flint) and var. testacea (Flint), which have 
no corresponding development in Rhiimbler’s life-cycle. 
Stages V. to VII. represent the complete life-cycle of Saccammina spherica 
(Sars), so far as it is a shell-bearing organism. The smallest specimens which we 
find (0:3 to 0-4 mm.) do not present any less difference in size, as compared with 
the adult, than is observable in the majority of rhizopods, and there is no reason 
for supposing that Saccammina spherica, whatever may be the earliest stages 
of its life-history, at present unknown, is in its shell-bearing stages in any way 
different from the adult, except in its smaller size, its somewhat rougher and 
less finished exterior, and in the fact that its general aperture is in the earliest 
stages a mere chink or fissure, which gradually develops into the nipple-like 
protuberance on which the aperture of the adult is placed. 
We present in support of our contentions a chart showing the relative distribu- 
tion of Psammosphera and Saccammina in the area examined, and a series of 
drawings showing all the stages in the development of the three organisms 
involved. 
19. The Relation of the Mechanics of the Cell to the Mechanics of 
Development. By Professor Dr. Lupwia RutmBuer. 
W. Roux has summarised the aim of the study of developmental mechanics 
in a single phrase, namely, ‘the mechanistic explanation of developments.’ The 
terms ‘mechanics’ and ‘mechanistic’ are to be understood here in the most 
general philosophical sense of the doctrine of the mechanistic course of events— 
that is to say, of events as subject to causality. The science of developmental 
mechanics has, therefore, to establish those factors which determine the path 
taken by the development of the living being in its course from the egg to the 
end of the embryonic life, quite irrespective of the nature of those factors, 
whether chemical, physical, or of any other nature whatsoever. 
It is obvious that within a field of research of such wide extent as that 
covered by developmental mechanics according to this definition, special lines 
of research will be developed, which, by limiting their scope so as to investigate 
certain definite modes of activity, while leaving unexplained other manifesta- 
tions of the organic activity, lighten their self-imposed task and strive to attain 
exact results. Such a side-branch of developmental mechanics is that which 
deals with the purely physical aspect of the processes of development—that is to 
say, which puts on one side the chemical composition of the embryonic body, and 
takes into consideration only the physical factors which come into action in the 
displacements and rearrangements during the process of organic form-produc- 
tion. This special branch of developmental mechanics may be termed develop- 
mental dynamics ; it comprises ‘the physics of the processes of modification and 
determination of form in the embryo.’ 4 
It has never been disputed from any side that physics and dynamics of this 
kind are in operation during the course of embryonic development, not even by 
_ the neo-vitalist Driesch ; for where masses are moved and arranged, the laws of 
the movements of masses—that is to say, of dynamics—must be operative. The 
application of physical laws to the living substance in process of development 
must, however, break down at the outset and lead to no result, if the physics of 
this substance were dependent in a specific manner upon the chemical composition 
of the masses in active or passive movement—if, that is to say, every chemically 
different substance also had its own distinct physical laws; for then it would be 
confronted by the whole chaos of our ignorance with regard to the chemical 
structure and mode of activity of the living substance. Fortunately, however, 
this is not the case; dynamic similarity or dynamic equivalence of a system does 
not necessitate in any way chemical similarity or chemical equivalence. A 
locomotive could be constructed with gold or any other rigid substance possessing 
the required physical properties instead of with iron, without altering the 
dynamics of the machine thereby ; on the contrary, it is evident that the dynamics 
of the processes in operation are dependent solely upon the aggregate condition, 
the mass-relations, and the arrangement of the constituent parts of the 
mechanical system which is carrying on the processes under consideration. 
KE 2 
