104 W. BLAXLAND BENHAM. 



one sees a double outline to the single membrane (such as 

 Boveri represents, PI. 6, fig. 8) or two closely apposed mem- 

 branes. 



Spengel vehemently animadverts on Lankester's interpre- 

 tation of this membrane as a mesoblastic cutis, and insists on 

 its being a "basement membrane/^ i.e. epiblastic. Lankester 

 was, it seems to me, in error in referring the origin of this 

 membrane to the deepest layer of nuclei in the sides of the 

 bar ; at the same time, if we consider the relations of this 

 membrane to the rod and to the vessels at each end, we cannot 

 doubt that the rod and the membrane have the same origin. 

 The rod would scarcely, I presume, be referred by Spengel 

 to the epiblast. I believe, however, that I have decided that 

 the rod is mesoblastic by the discovery of the flattened nuclei 

 pressed against its inner surface (PL 7, fig. 13) ; and we may 

 conclude that the rods in both bars are produced by the 

 flattened epithelium which, as Hatschek has pointed out, forms 

 the " connective tissue " throughout the body of Amphioxus. 

 If the rod is mesoblastic, then a priori "we may believe the 

 eptal membrane, which is absolutely continuous with it, to 

 be also mesoblastic ; but further, I have detected flattened 

 nuclei in this axis of the bar, i. e. between the two halves 

 of the closely apposed membranes. 1 searched long and care- 

 fully for any nuclei in relation to the septal membrane, and 

 ultimately, in my series of accurately transverse sections, I 

 was able, with the aid of Zeiss's apochromatic, to observe some 

 structures which I believed to be nuclei. However, I was not 

 absolutely certain of their existence, owing to the refrangibility 

 of the membrane and the denseness with which the epithelial 

 nuclei are packed ; but in a series of sections intended to be 

 horizontal, and stained in hsematoxylin, some of the bars were 

 cut longitudinally for a considerable distance — some six or 

 seven times the length of an ordinary transverse section of a bar, 

 — and here I saw distinctly elongated and much compressed 

 nuclei (fig. 21) of fair size, lying between the two membranes. 

 These, like the nuclei surrounding coelomic spaces, are not 

 rich in chromatin, and do not take the stain easily. We may. 



