Tract of modified Epithelium in the Embryo of Sepia. 5 9 



free margin (fig. 2, m.e., I). The middle one extends somewhat 

 farther forward than the other two, its anterior extremity 

 being about on a level with the back of the ink sac. At 

 their posterior ends the two branches which lie upon the 

 fins pass on to the body, and unite with the remaining one 

 in the dorsal median line. 



These three ridges are clearly visible by the naked eye in 

 embryos 5 to 8 mm. long, and appear to have been noticed by 

 Kolliker, who figures them in his classic monograph on the 

 " Embryology of the Cephalopoda." ^ He does not, however, 

 appear to describe them in the text, but merely alludes to 

 tliem in the explanation of the plates in the following 

 words: — "Die .... Doppellinien sind Leistchen, die durch 

 Hervorraounoen der Schale bewirkt werden." It will 

 abundantly appear from the sequel that this explanation 

 is erroneous ; there are no prominences on the shell of any 

 kind, and furthermore two of the lines in question are 

 situated not over the shell but on the fins. 



When seen in section, each of the lateral branches (fig. 3) 

 has the form of a more or less irregular ellipse, the length 

 of the major axis depending upon the obliquity of the 

 section. The cells of which it is composed are larger in all 

 dimensions than those of the neighbouring epithelium, so 

 that the level of the body-surface is slightly elevated where 

 they are present. The breadth of the tract is about Ol mm., 

 and the extreme elevation of the cells wdiich compose it is 

 about 0-075 mm. These are broader at the base than at the 

 free extremities, so that the lines bounding them converge as 

 they pass outwards. Their nuclei are spherical or slightly 

 ovoid, and in almost all cases provided with nucleoli. They 

 are situated almost exclusively near the bases of the cells, 

 though in one or two cases I obtained evidences of their 

 presence in the distal portions of the structure. 



The contents of the cells are finely granular, and stain more 

 deeply than those of the normal epithelium. The cell-boun- 

 daries become indefinite towards the superficies, and in some 

 cases this part of the tract stains more deeply than the rest. 



The medio-dorsal portion of the tract (fig. 4) is decidedly 



^ Entwickelungsgeschichte der Cephalopodeii, Taf. iii., fig. 32. 



