292 K- KISHINOUYE. 



dermic somite.s of many preceding segments and that they are entirely 

 separate from the ectoderm. Pi-evious authors who have studied the 

 development of the spider, overlooked this cavity in the mesoderm of 

 the caudal lobe, and observing the stnge at which the unpaired cavity 

 communicates with the proctodaBum consider the former as a portion 

 of the latter. If the cells enclosing the unpaired cavity are ectodermic 

 in origin, the numerous mesoderm cells in the caudal lobe must disap- 

 pear all at once, as there are no cells in the lobe except those surround- 

 ing the last fused pair of cœlomic cavities. But the disappearance of 

 many cells at once is cjuite impossible. 



As the process of reversion proceeds still further, each half of 

 the pair of the f.'celomic cavities in the cephalic lobe is divided into two 

 portions — one at the side of the stomodaeum (Figs. 9, 10, 1 ceci, h), 

 the other below the anterior border of the semicircular groove of the 

 brain (Figs. 9, 10, i cœl. a). The former disappears soon afterwards 

 but the latter elongates towards the dorsal median line (Fig. 11), and 

 the mesodermic walls of the cavities of two sides meeting nt the median 

 line fuse together, leaving, however, a canal between them. This canal 

 is the aorta. 



The portion of the egg which is not covered l)y the ventral plate 

 or the abdominal terga is characterised by the absence of the meso- 

 derm cells and by the presence of the secondary endoderm cells or the 

 fat cells instead. The secondary endoderm cells are directly under 

 the ectoderm. Most of them are enclosed between the walls of the 

 mesodermic moieties of the abdominal somites and becojne the blood 

 corpuscles. 



The cœlomic cavities in the segments of the chelicer£e and the 

 pedipalpi degenerate and disappear. The greater portion of the cœlo- 

 mic cavities of the four ambulatory appendages degenerates, the meso- 

 derm cells forming their wall becoming gradually changed into muscles. 



