446 SUMMARY OF CURRENT RESEARCHES RELATING TO 



cellensis Schrot. He finds that the walls and folds of the kidney consist 

 of fibrillar lacunar connective tissue in which are embedded elastic and 

 muscle-fibres, and of a single epithelial layer of nephridial cells, similar 

 in the sac and coils, and only slightly different in the duct. The ureter 

 and funnels have a specially differentiated epithelium. The connective 

 tissue lies as a very fine layer beneath the homogeneous epithelium. The 

 cylindrical nephridial cells have no specially developed basal membrane. 

 The plasma shows a fine reticular structure ; the nucleus with well- 

 defined nucleolus is median. The cells have flagella and fine hair-like 

 processes. They contain concretions of urine which are the final pro- 

 duct of the secretion of the cells. These concretions are circular, or 

 irregularly crystalline in form, and may number from two to thirty. 

 The smaller crystals are sometimes aggregated in masses. The nephridial 

 cells also contain granules, circular formations about the size of a 

 nucleolus, disposed in definite longitudinal series along the lateral walls 

 of the basal portion of the cell. They are probably to be regarded as 

 the " plasmosomes " of Duesberg. The distal portion of the cell often 

 contained vacuoles, but these could not be identified with the " secretory 

 vesicles " of Leydig. Mitotic cell-multiplication occurs in the epithelial 

 cells, but mitosis does not appear to affect the secretion process 

 materially. 



Arthropoda. 



o. Insecta. 



Optic Ganglia of Dragon-fly Larvae.* — Alexius Zawarzin has made 

 a detailed study of the minute structure of the three optic ganglia in 

 sEschna larva?. The very striking differentiation in layers, which Radl 

 described, is discussed at length. Among the cells surrounding the 

 medullary substance of each ganglion there are three types. Some send 

 a process through the medullary substance, giving off lateral branches, 

 and passing to another ganglion. Some send a process into the me- 

 dullary substance, which passes out again and enters another ganglion. 

 Some have a process which does not pass out of the ganglion to which 

 they belong. Other cells which usually lie to the side of a ganglion may 

 be called the cells of the horizontal plexus, and they, again, exhibit 

 various types. 



The first ganglion is simplest. Its medullary substance has only 

 three layers. In the second there are eighteen, in the third twenty-one 

 layers. The structure is extraordinarily complicated. In Insects the 

 fundamental chain of elements has four links — (1) retina cells ; (2) cells 

 with processes passing through the first ganglion ; (3) similar cells 

 passing through the second ganglion ; and (4) the internal cells of the 

 third ganglion. These may be compared with the chain in Vertebrates 

 — (1) optic cells ; (2) bipolar cells ; (3) ganglion-cells ; and (4) cells of 

 the optic lobes with long processes. The first two elements in the chain 

 may be compared to the optic cells of Cephalopods, with their long 

 processes ending in the plexiform layers ; the other two elements are 

 represented by the cells of the inner granular layer, and the cells of the 

 medullary layer with descending processes. 



* Zeitschr. wiss. Zool., cviii. (1914) pp. 175-257 (6 pis. and 19 figs.). 



