THEORIES OF THE CCELOM 17 



higher animals. Leuckart had in 1848 maintained that the 

 alimentary canal and the body -cavity of higher animals were 

 united in one system of cavities in the Enterocoela (Verwandtschaf ts- 

 verhaltnisse der wirbellosen Thiere, Brunswick, 1848). Metschni- 

 koff insisted upon such a correspondence when comparing the 

 Echinoderm lai /a, with its still continuous enteron and ccelom, to 

 a Ctenophor, with its permanently continuous system of cavities 

 and canals. Kowalewsky in 1871 showed that the body-cavity of 

 Sagitta was formed by a division of the archenteron (Fig. 5) into 

 three parallel cavities, and in 1874 demonstrated the same fact for 

 the Brachiopoda (see Fig. 3). 



In 1875 (Quart. Journ. Micr. Sci. vol. xv. p. 52) Huxley 

 proposed to distinguish three kinds of body-cavity : the schizoccel, 



FIG. 5. THREE STAGES (A, B, (J) IN THE DEVELOPMENT OF SAGITTA TO SHOW THE ORIGIN 



OF THE CCELOM AS A PAIR OF ENTEROCCELOUS POUCHES. 



m, mouth ; oJ, alimentary canal ; a*, archenteron ; bl.p, blastopore ; pv, cnelomic pouch ; 

 so, sp, epithelial wall of the same pouch ; ge, gonad cells. (After Butschli and Kowalewsky, 

 from Balfour.) 



formed by a splitting of the mesoblast, as in the chick's blasto- 

 derm ; the enterocoel, formed by pouching of the archenteron, as 

 in Echinoderms, Sagitta and Brachiopoda ; and the epico3l. This 

 last name he applied to the atrial chamber of Tunicates and to 

 a supposed chamber in Amphioxus, the existence of which he was 

 led to believe in, by the examination of ill-preserved specimens. 



Immediately after this I put forward the theory of the 

 uniformity of origin of the coelom as an enteroco?! (Quart. Journ. 

 Micr. Sci. April 1875). I pointed out that inasmuch as it had 

 been shown in many cases that the mesoblast is derived from the 

 hypoblast (wall of the archenteron), it might well be supposed 

 that the splitting of the mesoblast is only a delayed formation of 

 the lumen of the enterocoelous pouch : that in fact the mesoblastic 

 somites and solid paired masses are only enteroco3l pouches in a 

 solid condition, destined after a brief delay to open out as pouches 

 or sacs. My theory of the ccelom as an enterocoel was accepted 



