ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
31 
while the optic power of spherical surfaces is largely dependent on the 
surrounding medium. 
Exner finds that a cone of the eye of a Limulus, from its cornea to 
its tip, corresponds to a dioptric apparatus which acts essentially like a 
lens-cylinder, and that the more the greater its focal length. As the 
retinula is surrounded by pigment it only receives light from a single 
cone, which has an optic area of about 8°. From the optic area of all 
the separate facets the general optic area of the eye is made up. This 
is an “ appositionsbild ” in contradistinction to a “ superpositionsbild.” 
The latter is characterized by rays of light belonging to several facets 
falling largely over one another. In Lampyris splendidula (fig. 2) the 
Fia. 2. 
cornea-cylinders are separated from the retina (R) by a broad inter- 
mediate layer, and the pigment does not, as in the eye of Limulus, 
surround a single facet-segment, but one and the same part of the 
retina may receive light from various cornea-cylinders. This gives rise 
to the superposition image, which is upright. 
Of the many interesting points dealt with we can only call attention 
to the much vexed question as to the powers of facetted eyes ; at a 
distance of one centimetre Lampyris would be able to distinguish the 
bar of a network not more than O’ 22 mm. broad. And in others the 
sense of sight is even keener. 
Neuroblasts in Arthropod Embryos.* — Mr. W. M. Wheeler has a 
preliminary notice of the results of his investigation of the develop- 
ment of the nervous system in Xiphidium ensiferum. The nervous 
system makes its appearance early, the ganglia arising as paired 
thickenings of the ectoderm in the manner so often described for Arthro- 
pods. Transverse sections through either lateral cord are seen to 
consist, in early stages, of two kinds of ectodermal elements ; the smaller 
cells with rather deeply stainable elongate oval nuclei, while four larger 
cells have pale spherical nuclei. These four cells are the neuroblasts ; 
* Journal of Morphology, iv. (1891) pp. 337-43. 
