SOOLOGY AND BOTANY, MICROSCOPYj ETC. 499 



the individual animals enter it, there are numerous much-hranched 

 diverticula of the ectodermal processes. These bud-foundations form 

 a conical elevation in the middle of the head ; and the development of 

 the buds seems to take place as described by Kowalevsky in Didemnum 

 styliferum and Amouromium. 



Arthropoda. 

 cs. Insecta. 



Histology and Development of Insects.*— H. Viallanes finds 

 that, in the larva, the visual apparatus consists of three principal 

 parts : the imaginal disk of the eye, the nervous tract, and the optic 

 ganglion. The first of these has the same structure as all other disks 

 of the same kind, or, in other words, consists of a provisional, an 

 ectodermal, and a mesodermal layer. Some time before the meta- 

 morphosis the most superficial cells of the ectoderm increase in size 

 and become elongated, while at the same time they acquire the 

 property of being coloured in a particularly intense manner under 

 the influence of certain staining reagents ; and it is at this time that 

 they become "optogenous cells." The mesoderm of the optic disk 

 has not the structure of the same layer in other disks. It is not 

 formed by a fundamental homogeneous substance connecting the 

 cells, but rather by fine nervous fibrils, with which nuclei are inter- 

 mixed, and which appear to end in the basal membrane of the 

 ectoderm, but really pass through it to become continuous with the 

 extremity of an optogenous cell. The nervous tract is formed by 

 fine nervous fibrils continuous with those of the mesoderm ; so that, 

 in other words, the mesoderm of the disk is only an enlargement of 

 the nerve-tract ; and thus, when differentiation is complete, we find 

 that each optogenous cell is connected with the nervous centre. 



The optic ganglion is formed by the outermost portion of the 

 cerebral ganglion, and is invested by a neurilemma. On the lateral 

 parts of the optic ganglion and in the grey cortex there is a very 

 complex organ, which may well be called the rudiment of the 

 ganglionic layer, for in this we may see all the chief parts which 

 enter into the constitution of the definite ganglionic layer ; externally 

 there is the layer of ganglionic cells, which, as in the adult, are 

 formed of bipolar cells united to form short rows. Just within this 

 there is an indication of the layer of palisade-like fibres, then a layer 

 of fibres and nuclei, the last being the rudiment of the layer of 

 nucleated fibres. The fibres of the nervous tract arise from the 

 surface of the layer of ganglionic cells, just as the post-retinal fibres 

 do in the adult. The only important difference between the chief 

 parts of the optic apparatus in the larva and in the adult would 

 appear to be in the more compact condition which obtains in the 

 former. 



When metamorphosis occurs the provisional layer of the imaginal 

 disk of the eye disappears, the ectoderm increases in size, and developes 

 a membrane, while its edges become united with those of the neigh- 



* Ann. Sci. Nat. (Zool.) xiv. (1882) pp, 1-348 (18 pis.). 



2 K 2 



