502 ADAM SEDGWICK. 



Kennel's description of the origin of the heart and peri- 

 cardium from an irregular system of spaces in the dorsal 

 mesoderm, derived from the broken-up dorsal divisions of the 

 somites, is quite erroneous. 



It is easy to see the arrangements which I have described, 

 if embryos be used in which the dorsal ectoderm has not con- 

 tracted. 



Corrosive sublimate solution with a few drops of acetic, or 

 Perenyi's fluid, or 70 per cent, spirit, seem to be the best 

 reagents for obtaining this result, though their action is by no 

 means certain. 



With regard to the fate of the two new cavities the dorsal 

 of them [b. pc.) becomes the pericardium (PL XXXVII, figs. 

 45, 46); the ventral {b. be.) enlarges, and by the withdrawal of 

 the gut from the ventral wall of the dorsal division of the somite 

 — which aborts, it will be remembereri, in the anterior region 

 (PI. XXXVII, fig. 46) but gives rise to the generative organs 

 in the posterior (PI. XXXVII, fig. 43) — becomes continuous 

 with its fellow. It remains for some time separated from the 

 ventral median chamber {b. be.) by the septum, which results 

 from its method of origin. This septum eventually — Stage g — 

 breaks down and the cavity b. be' . becomes continuous with b. be 

 and the two form one chamber, the median chamber of the 

 definite body cavity of the adult. 



I have now described the origin of all the parts of the adult 

 body cavity except those in the legs. These arise in Stage f 

 as spaces in the thickened mesoderm of the appendages 

 (Pi. XXXVII, figs. 52, 53, b. app). 



From the above account it is perfectly clear that, so far as its 

 embryonic development is concerned, the body cavity of Peri- 

 pat us has, as I stated at the outset, nothing to do with the 

 coelom. It is a pseudocoele, a space which arises secondarily, 

 i. e. subsequently to the coelom, partly in the mesoderm masses 

 produced from the walls of the somites and partly as spaces 

 between the ectoderm and endoderm (cf. the vacuoles in the 

 same position in the gastrula stage), which soon become lined 

 by mesoderm cells from the somites. The heart has an origin 



