86 The American Naturalist. l [January,. 
alveolar rows finally break in the cleavage of the cytoplasm. Mean- 
while the vacuole-like nucleus has been pulled apart towards the centro- 
somes. 
After the cleavage of the cell the centrosome again ceases to absorb 
liquid and so passes into the resting stage. The case where there is an 
immediate division of the centrosome into two that remain for the sub- 
sequent cell divisions also admits of explanation upon this alveolar 
basis; the same is true of various other cases and phenomena. 
If this attempt at an understanding of the complex marvels of cell 
division appears much too ineffectual it is partly due to the imperfect 
representation given in this abstract and in part the result of the un- 
finished character of the present paper which claims to be but the first 
of a series. In the next article the author hopes to consider the nucleus 
with its chromosomes and spindle; and we cannot well judge of the 
success or failure of the attempt till that part lies before us. 
Probably few will judge that much ultimate truth has yet been dis- 
covered in the attempted explanation of such exceedingly complex phe- 
nomena but if it prove that the right line of research has been struck 
the author has added much to our conceptions of the forces at work in 
embryological processes. 
PSYCHOLOGY.’ 
Experiment on Reinversion of the Retinal Image.—The 
inversion of the image on the retina, and its influence upon our visual 
perception of space, have given rise to considerable discussion in the 
past. That we see things in an upright position notwithstanding this 
inversion, has seemed to many writers to require special explanation- 
Accordingly, some have assumed a reinversion of the image in the — 
cerebral cortex, while others have adopted a theory of visual projection 
which makes the retinal inversion essential to upright vision. During 
all this discussion the possible relativity of up and down escaped notice 
for a long time. What we mean by down is simply the ground side, 
and by up simply the sky side. As everything imaged on the retina is 
inverted, there is no point of reference to give an indication of the 
inversion of the rest. The only problem that arises, then, is concern- 
ing the co-ordination of visual with tactile space. This is a real diffi- 
culty: I see my left hand down at my left side; I feel it in the same 
place. How can this co-ordination be reconciled with the fact of retinal 
1 Edited by H. C. Warren, Princeton University, Princeton, N. J. 
