364 DISCOVERY REPORTS 



(b) The photogenic streak behind the fifth thoracic limb 

 Kemp (igiob, p. 644) describes the transverse photogenic streak behind the last 

 thoracic limb of S. debilis as consisting of elongated epithelial cells lying beneath a lens- 

 like thickening of the integument. The epithelial cells usually, though not always, have 

 their nuclei placed close behind the lens. No granules similar to those at the bases of 

 the photogenic cells of the pleopod organs are mentioned. It is not clear from the 

 description whether the elongated epithelial cells, "as in the photophores on the 

 pleopods", have actually the same curious structure as those of the pleopod photo- 

 phores, or whether they are merely elongated but otherwise normal epithelial cells. 



Whereas the sections studied by Kemp were cut transversely to the photogenic 

 streak, those I have examined were cut longitudinally in order to include as much of the 

 photogenic tissue as possible in the section. My sections show that the photogenic 

 elements consist of cells resembling those of the pleopod photophores and almost 

 identical with those of the corresponding transverse streak of S. affinis (p. 354 and 

 Fig. 25). The identity of the cells with those of S. affinis is so marked as not to require 

 illustration. Their nuclei are distally placed, and lie close beneath the lens. They are 

 characteristically truncate, presenting a plane or slightly concave inner surface. No 

 cytoplasmic contents are apparent, but basally each cell rests in a cup of non-staining 

 granular substance (compare with Fig. 25, pr.gr. m.). No basal cap, as in S. affitiis, was, 

 however, apparent at the basal tip of the cells. The photogenic cells are disposed in a 

 number of closely parallel linear series, about four rows of cells usually forming the 

 width of the streak. 



I believe that the discrepancy between the account given by Kemp and my state- 

 ments above is not a serious one, however, and that they can be reconciled in the 

 following manner. A careful study of the photograph submitted by Kemp (1910&, 

 pi. Hi, fig. 2) reveals the presence of two nuclei whose shape is characteristic of the 

 photogenic nuclei I have described above and also of those seen in the pleopod photo- 

 phores. They are the sixth and seventh nuclei from the left-hand margin of the lens, 

 and are considerably broader than those surrounding them. On their inner side there 

 is a clear area not seen elsewhere in the same plane. I am of the opinion that these 

 nuclei are those of the photogenic cells so numerous in my sections, and that the clear 

 space beneath them represents that part of the cells which is devoid of cytoplasm. All 

 adjacent nuclei appear to be those of the normal chitogenous epithelium lying to right 

 and left of the photogenic streak. This view is supported by measurement of the 

 photogenic nuclei: both those in Kemp's photograph and in my sections appear to be 

 about io/j,m breath. Additionally, the relation between the length of the photogenic 

 nuclei and the thickness of the lens is about the same in each case. Further than this 

 it is not possible to go on the information given by Kemp, but the different views 

 obtained from sections cut transversely and those cut longitudinally may well account 

 for the difference between the two accounts. 



The presence in the transverse streak of photogenic cells of the type found in the 



