NOTE <")N THE CtELOMIC CAVITY OF THE SPIDER. 291 



also that tit firwt " the only dor.sal element« developed were the live 

 pah'8 belonging to the abdominal somite« («econd to «ixtli), but during 

 this stage the dorsal elements of the limb-bearing somites begin a more 

 rapid growth." But according to my observation this is not the case, 

 the dorsal elements of the limb-bearing somites remain undeveloped as 

 in the pre\ious stage. 



Between two consecutive terga there is a furrow. This furrow 

 was described by Schimkewitsch* as ha\ ing no relation ^^ ith the meso- 

 dermic somites. He says that it is due to the niesodermic somites 

 fusing together, before they develop dorsally to cover the dorsal sur- 

 face. He says moreover that the number of these furrows never 

 correspcjnds with that of the somites. My observations as given above 

 do not corroborate these statements. 



The last three abdominal segments (sixth to eighth) gradually 

 deii'cnerate and their cœlomic cavities seem to fuse tooether into one 

 pair. The pair of the cd'lomic cavities thus formed by fusion is pushed 

 into the protuberance of the tail as the process of reversion proceeds 

 (Fig. 7). I find tliat fusion of the cœlomic cavities does Mot take 

 place before this stage though Schimkewitsch says that it does in the 

 céphalothorax and in the alxlomen and I also erroneously stated that it 

 occurs in the thorax before tliis stage. 



A cavity is produced in the mesoderm of the tail lobe. It is 

 iüi])aired (Figs. 7, (S). The unpaired cavity tlius made cannot be con- 

 ceived otherwise than as a homologue of the ccelomic cavity. Though 

 the cavity is certainly not formed by ;in invagination, I thought that 

 the cells in the tail lobe might be produced by the proliferation of the 

 ectoderm. But I found that the cells enclosing the unpaired cavity 

 are the remnant of the mesoderm cells which gave rise to the meso- 



* loc. cit. 



