DEVELOPMENT OF ELASSIOBRANCH FISHES. 13 



The furrows themselves for the most part are by no means 

 simple slits with parallel sides. They exhibit a beaded structure, 

 shewn imperfectly in PL I. fig. 6, but better in PI. I. fig. 6 a, 

 ■which is executed on a larger scale. They present intervals 

 of dilatations where the protoplasms of the segments on the 

 tv/o sides of the furrow are widely separated, alternating with 

 intervals where the protoplasms of the two segments are almost 

 in contact and are only separated from one another by a very 

 narrow space. 



A closer study of the germinal disc at this period shews 

 that the cavities which cause the beaded structure of the 

 furrows are not only present along the lines of the furrows 

 but are also found scattered generally through the germinal disc, 

 though far more thickly in the neighbourhood of the furrows. 

 Their appearance is that of vacuoles, and with these they are 

 probably to be compared. There can be little question that 

 in the living germinal disc they are filled with fluid. In some 

 cases, they are collected in very large numbers in the region 

 of a furrow. Such a case as this is shewn in PI. I. fig. 6 h. In 

 numerous other cases they occur, roughly speaking, alternately 

 on each side of a furrow. Some furrows, though not many, are 

 entirely destitute of these structures. The character of their 

 distribution renders it impossible to overlook the fact that these 

 vacuole-like bodies have important relations with the formation 

 of the segmentation furrow\s. 



Lining the two sides of the segmentation furrows there is 

 present in sections a layer which stains deeply with colouring 

 re-agents; and the surface of the blastoderm is stained in the 

 same manner. In neither case is it permissible to suppose 

 that any membrane-like structure is present. In many cases a 

 similar very delicate, but deeply-stained line, invests the vacuo- 

 lar cavities, but the fluid fiUing these remains quite unstained. 

 When distinct segments are formed, each of these is surrounded 

 by a similarly stained line. 



The yolk-spherules are so numerous, and render even the 

 thinnest section so opaque, that I have failed to make satis- 

 factory observations on the behaviour of the nucleus. I find 

 nuclei in many of the segments, though it is very difficult even 

 to see them, and only in very favourable specimens can their 



