SEGMENTATION. 225 



observed an example of two furrows crossing one another at 

 this stage. 



The furrows themselves for the most part are by no means 

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

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

 which is executed on a larger scale. They present intervals 

 of dilatations where the protoplasms of the segments on the 

 two 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 ap- 

 pearance 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. 6, fig. 6 b. 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 seg- 

 mentation furrows. 



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 filling 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 



