ZOOLOGY AND BOTANY, MICROSCOPY, EIC. 35 



appearance of the stomodce-urn. There are now three layer?, all of which 

 are concerned in the development of the optic apparatus ; the outermost 

 is the epiblast, and the two others are derived from the invaginated 

 portion of the same layer. The innermost may be called the gangliogen, 

 as it will give rise to the chain of ganglia and nerves which lies within 

 the stalk of the adult eye, and connects the optic apparatus with the 

 brain. The middle layer — which may be called the retinogen — will 

 give rise to all the retinal parts of the eye. 



Borne complex changes in the appearance of the cells are brought 

 by the mode of division of their nuclei ; after a time it will be 

 found that the ectodermal nuclei has come to correspond with those of 

 i rlying layer, and that the nuclei of the retinogen and gangliogen 

 have each given rise to five nuclei arranged in a row, while the rows are 

 arranged in Bets. In section two will be seen closely appressed to each 

 other, and separated from the adjacent parts by a rod of apparently 

 str uc tu reless material ; this last is the rudiment of the crystalline cone, 

 and the adjacent rows of nuclei belong to different ommatidia, or optic 

 elements. In the ganglionic layer the rows of nuclei have broken, and 

 formed the rudiments of two ganglia. 



In a later stage the epidermis-cells will be seen to be distinct from 

 ' - of the retinogen and to have become the cuticle, which is modified 

 into lenses over each crystalline cone. Development and differentiation 

 have gone on in the rows of retinal nuclei, each of the cells having become 

 greatly elongated, the protoplasm extending out to a considerable distance 

 : the nucleus in a thread-like prolongation ; the nuclei are placed 



at different heights in those cells, and the tail-like prolongations are 

 arranged in layers around the crystalline cone ; the distal cell of the 

 retinal row is clearly the crystalline cone-cell or retinophora. Four of 

 these surround the cone, and their walls so touch that they form a cup 

 in which the cone is situated, and from which it is secreted. Eelow the 

 calyx the ends of the retinophoral cells unite to form a slender pedicle, 

 which is clearly the rhab lom of Grenadier, and which is, as clearly, 

 formed by the retinoph jree. and is not a secretion from the surrounding 

 pigment-cells. 



As to the phylogeny of the Arthropod eye, we may suppose that the 

 invaginated pit had sensory functions, and either wall must, for a time, 

 have been like its fellow, as is shown by its having similar nuclei, and 

 by the similar development of rows of nuclei. The position of the eye 

 at the extreme ends of the nervous cords would indicate that it was 

 differentiated as part of the primitive nervous system; but it is not yet 

 to be said that the invagination was confined to the eye alone, and did 

 not extend through the whole length of the cords ; on this question the 

 fact that the supra-oesophageal commissure developes much later than 

 the optic cords may be of significance. 



'Challenger 1 Cumacea.* — Prof. G. 0. Sars commences his account 

 of the Cumacea collected by H.il.S. ; Challenger,' by considering their 

 morphology. He cannot agree with Boas in regarding them as very 

 nearly related to the Mysidse, but thinks they represent an isolated branch, 

 which cann.t strictly be derived from any of the recent groups; it is 

 thai -.me of the palaeozoic Phyliocarids formed a direct trans- 

 ition to the Cumacean type. 



* Rf-porta of the Vovage of H.M.S. ' Challenger,' lv. (1SS7) 78 pp., 11 pis 



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