ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 209 



distinctly divided into two lateral lobes ; it is formed of a granulo- 

 fibrillar mass enveloped in a nucleated cortex. Several pairs of nerves 

 are given off symmetrically from its surface. In P. benedeni the brain 

 is always in front of the ovaries, in P. echinata it is partly between 

 them, and in P. bchjica it is completely surrounded by them. 



Arthropoda. 



Eyes of Arthropods.* — Dr. W. Patten, finding that his observations 

 on the structure of the adult eyes of insects differed widely from those 

 of other recent writers, has endeavoured to confirm, by embryological 

 data, the continuity of the so-called rhabdom with the crystalline-cone 

 cells, and also his observations on the nature of the corneal hypodermis, 

 or, as he now prefers to call it, the corneagen. As the compound eye and 

 optic ganglion of Vespa develope slowly, and the successive stages are 

 clearly defined, it is an admirable subject for investigation. 



Among the more important points which this investigation has 

 brought out are — 



(1) The crystalline-cone cells, or any of the eventually pigmented 

 cells surrounding them, do not form a layer of cells distinct from and 

 superimposed on the retinulae ; for the crystalline-cone cells, the retinulae, 

 and the other pigmented cells are derived from, and remain a single 

 layer of cells. 



(2) The rhabdom is not a product of the retinulse, but is merely the 

 inward prolongation, or stalk, of the crystalline-cone cells. 



(3) The layer of cells from which the ommateum arises is the inner 

 wall of an optic vesicle formed by an invagination of the ectoderm, and 

 the ommateal cells are consequently upright. 



(4) The retinophorae which, in the adult, are grouped in fours, are in 

 the youngest stages arranged in twos, or repeat the permanent con- 

 dition of the retinophorse in the ocelli of most insects, and in the simpler 

 compound eyes of Crustacea. 



(5) The pigment first appears in the form of paired patches around 

 the paired retinophorse, and is retained until after the retinophorse have 

 increased to four. This transitory condition of the ommatidiai cells in 

 the compound eye probably corresponds with the permanently paired 

 arrangement of the pigment patches and retinophorse of the ocelli. At 

 the commencement of the pupal stage the eye consists of three layers, the 

 innermost being the ommateum, the middle layer being composed of 

 cells containing large round nuclei, arranged at regular intervals over 

 the retinophorse, and the third of flattened cells with quite small nuclei ; 

 the last is but slightly modified in the adult, where it forms the corneagen. 

 The cells of the middle layer become sickle-shaped and arrange them- 

 selves in pairs, a single cell on either side of a calyx. They grow 

 inwards as far as the neck of the calyx, where they terminate in a 

 rounded swelling containing a large nucleus ; their inner ends soon 

 become deeply pigmented, and appear to form a part of each ommatidium. 

 Surrounding the sickle-shaped cells are the ends of a circle of eighteen 

 more cells, so that each ommatidium, including the four retinophora) 

 but not the two middle-layer cells, is composed of twenty-two cells. 



The author is of opinion that the difference between his results 

 and those recently obtained by Reichenbach on Astacus is more one of 

 interpretation than of observation. His own observations and those of 



* Journ. of Morphology, i. (1SS7) pp. 103-226 (1 pi.). 



