4889-1 Polar Differentiation. 221 
those around its equator and still better than in those at its 
constant posterior pole. The diffusion or extension of the 
primordial visual apparatus of the protozoan grade such as is 
seen in Euglena, is a result merely, in Volvox, of the perma- 
nent attainment of the colonial grade of development which 
has ended in a sort of blastula-like form, each cell of which is 
provided with a sense organ. In other words we have in Vol- 
vox a blastula-like type with a sensory apparatus apparently 
developed at its anterior pole, while at its posterior pole this 
sensory apparatus is so little developed as to be nearly absent, 
possibly owing to disuse. The degree of development of this 
supposed sensiferous apparatus at opposite poles in Volvox 
stands in an obvious relation to the respective importance of 
such a contrivance at those poles in relation to the welfare 
of the organism. It is probable that, if what I have here des- 
cribed is really a visual or other sensory apparatus, it is the 
most primitive and unspecialized compound sensiferous organ 
yet detected in the living world. At any rate it is probably 
to be regarded as a compound organ in the same sense that 
the retina and ommatidia of other and higher forms are to be 
regarded as compound organs in that they are cellular aggre- 
gates. The further study of these remarkable structures and 
relations in Volvox is desirable, and as the organism is acces- 
sible to many students it is to be hoped that such study may 
not be long delayed, and that not only a more careful study 
of the minute structure of the "eye-spots" may be carried 
out, but also that figures will be produced which will give 
adequate prominence to the most important of the facts which 
I have here attempted to put upon record. 
THE DEVELOPMENT OF THE THEORIES 
OF CRYSTAL STRUCTURE.' 
JN 1822, the Abbe Hauy^ declared that since all crystals of 
the same substance, whatever their external form, may be 
1 Abstracted by. W. S. Bayley from an article by H. A. Miers in Nature of 
January 17, 1889. 
"^ "Traite de Cristallographie." (Paris, 1822.) 
