ON THE DEVELOPMENT OP ARANEINA. 67 



if they have just emcrg'cd from the yolk. In the second place, their 

 nuclei ao;ree in their loroe size with those found in the yolk. 



At the end of this stage the mesoderm in the caudal lobe is 

 faintly divided into two layers, between which an unpaired cavity 

 makes its appearance (PI. XIV, û^. 24). In the cephalic lobe also 

 the mesoderm is faintly divided into two layers on each side (PI. 

 XIV, fig-. 23), enclosing the rudiments of the cœlomic cavities. It 

 is still undivided in the median line. The cœlomic cavities in the 

 thorax secondarily fuse together into a single cavity. They remain, 

 however, quite distinct in the abdominal region. 



The Period of the Reversion of the Embryo. 



The stage in which the reversion of the embryo occurs is as 

 difficult to study as it is important, since many organs arise at the 

 same time. At the end of the last stage, the ventral plate had reached 

 the maximum limit of dorsal flexure, the cephalic and the anal lobes 

 almost toucl.dng each other (PI. XIV, figs. 24, 25). As Balfour 

 states, the reversion of the embryo is due to the expansion of the 

 dorsum ; and the expansion of the dorsum is due to the horizontal 

 increase of cells which compose that part. The head and the tail are 

 pushed away from one another further and further. As the dorsum 

 is very rapidly expanding and the cells are pressed for room, a groove 

 is produced immediately behind the tail lobe to increase the surface 

 of the dorsum, and the tail lobe then stands out as a conical process 

 (ri. XIV, figs. 26-29). The cœlomic cavities belonging to the 

 segment in front of the tail lobe being pressed from the dorsal side 

 by the increase of cells in the dorsum are compressed horizontally and 

 pushed into the conical tail process, enveloping the unpaired cœlomic 

 cavity of that process from the dorsal side. The caudal lobe stands 



