NOTE OX THE CŒLOMIC CAVITY OF THE SPIDER. 289 



by the inedijin partition of mesodermic ceJJs ; while the right nnd loft 

 cœlomic cavities of the other segments are separated by the yolk 

 (Fig8. 3, 5). 



The cephalothoracic appendages, with the co'lomic cavities in 

 them, elongate very much, and bend towards the ventral median line 

 (Fig. 5). The mesodermic somite and also the co^lomic cavities of 

 the first abdominal segment have developed little since their formation. 

 Korschelt and Heider state in their text book that in this seoinent a 

 pair of the provisional appendages is formed as in the four succeeding 

 seginents. Though a pair of slight elevations is found in this segment, 

 tliey are very much lower than the ])rovisional appendages of the suc- 

 ceeding segments, and moreover they are chiefly of the ectodermic 

 thickening (Fig. 4). I am therefore inclined not to call them pro- 

 visional appendages. In each of the second to fifth abdominal seg- 

 ments, a pair of provisional appendages appears. They are short 

 elevations of the ectoderm, into which the cœlomic cavity enters, as in 

 the case of the cephalothoracic appendages. The C(Elomic cavities of 

 these segments develop in the dorsal direction : hence in cross 

 sections of the embryo these abdominal somites differ from the thoracic 

 segments in having a shorter branch of the cœlom in the apj^endao-e, 

 and a longer one towards the dorsal side (Fig. ,5). The ectodermic 

 cells covering the mesoderm somites are always high and columnar 

 in shape, and are easily distinguished from the cells of other parts. 



Soon afterwards one more new segment is cut off from the 

 caudal lobe, and in its mesodermic moieties a pair of cœlomic cavities 

 appears. At this stage, the ventral plate attains the greatest antero- 

 posterior extension round the egg, so that the cephalic and the caudal 

 lobes almost touch each other (Figs, o, 6). The caudal lobe is raised 

 a little above the general surface of the egg. I stated erroneously in 

 my first paper that the mesoderm of the caudal lobe is split to form 



