i8 



THEORIES OF THE CCELOM 



by Balfour, and was greatly strengthened by his observations on 

 the derivation of both notochord and mesoblastic somites from 

 archenteron in the Elasmobranchs, and by the publication in 1877 

 by Kowalewsky of his second paper on the development of 

 Amphioxus in which the actual condition which I had supposed 

 to exist in the Vertebrata was shown to occur (see Figs. 6, 7, and 

 8), namely, the formation of the mesoblast as paired pouches in 

 which a narrow lumen exists, but is practically obliterated on the 



FIG. 6. 



ik 



dh 



FIG. 



FIG. 8., 

 FIGS. 6, 7, 8. TRANSVERSE SECTIONS OF 



THE BODY OF THREE LARV^: OF 



AMPHIOXUS AT SUCCESSIVE STAGES 



OF DEVELOPMENT IN -ORDER TO SHOW 



THE ORIGIN OF THE CCELOM AS 

 PAIRED ENTEROCCELOUS POUCHES. 



FIG. 6 shows the ccelomic pouches 

 (IK) as part of the enteric wall. 



FJG. 7 shows them nipped off as 

 closed sacs. 



FIG. 8 shows them pushing their 

 way between ectoderm and endoderm ; 

 the right-hand sac has divided into an 

 upper " myocozl " and a lower " splanch- 

 nocoel." at, ectoderm, ik, endodenn, 

 mk, wfci, mk 2 , epithelium of the coe- 

 lomic wall ; Ih, coclom ; mp, foundation 

 of the nerve cord ; n, nerve cord ; ch, 

 notochord ; us, myocoel ; dh, gut. (After 

 Hatschek, from Hertwig.) 



nipping off of the pouch from the archenteron, after which process 

 it opens out again as ccelom. 



The chief difficulty which my theory of the uniform nature of 

 the coelom had to encounter was in bringing the cavities con- 

 sidered to be " ccelom " in the Mollusca and the Arthropoda into 

 the scheme. At this time I accepted, in common with most embryo- 

 logists, the view of Haeckel and Gegenbaur, that the irregular 

 and more or less spongy space holding blood in those animals is 

 in reality the coelom, and as a part of that interpretation I accepted 

 the theory that the blood-vascular system is itself only a part of 

 the ccelom cut off from it and specialised in most cases, but con- 



