542 



CEPH ALOCHOBD A . 



ak 



then form paired coelomic sacs closed on all sides (Fig. 283, mk}. 

 The coelom consequently here forms as in Sagitta, Balanoylossus, the 

 Echinoderma and the Brachiopoda, through abstriction of pouches 

 (enterocoeles) from the walls of the archenteron ; the coelomic cavity 

 is derived from a part of the primary archenteric cavity. While the 

 anterior primitive segments develop more and more, new segments 

 are constantly becoming severed from the contiguous, unsegmented 

 part of the mesoderm-folds. 



LWOPF has recently described the formation of the coelomic sacs 

 in a way differing from that of KOWALEVSKY and HATSCHEK. 

 According to him, the development of the two mesoderm-folds has no 



direct connection with the rise of the 

 coelomic sacs, but results merely 

 mechanically as a consequence of the 

 sinking in of the medullary tube 

 between them. The primitive seg- 

 ments, indeed, arise through abstriction 

 from these folds, but the cavities found 

 in them disappear, while the coelom of 

 the adult rises only later through the 

 shifting apart of the cells composing 

 the temporarily solid primitive segment. 

 Even if LWOFF'S observations should 

 be confirmed, the way in which the 

 primitive segments here develop would 

 still have to be regarded as a modifica- 

 tion of development by evagination or 

 out-folding of the archenteron in the 

 sense given above.* 



The notochord or chorda dorsalis 

 develops somewhat later than the first 



rudiment of the medullary tube and the primitive segments, 

 arising through a folding in of that part of the dorsal wall of the 

 <3ntoderm-sac which extends between the mesoderm-folds (Fig. 282, ch). 

 This median entoderm-mass lying beneath the medullary plate at first 

 bulges into the archenteron dorsally (Fig. 282 B), but later changes 

 into an outgrowth (Fig. 283), the lumen of which is finally a mere 

 slit. Even this slit disappears later, and the cells which come into 



FIG. 284. Transverse section 

 through the middle of the body 

 of an embryo of Amphioxus with 

 eleven primitive segments. On 

 the left, only one primitive seg- 

 ment has been cut through, but 

 on the right, two consecutive 

 segments are seen, ak, ecto- 

 derm ; ch, rudiment of chorda ; 

 dh, enteric cavity; ik, entoderm ; 

 Ih, body-cavity ; mk 1 , somatic, 

 mk 2 , splanchnic layer of the 

 mesoderrn ; it, neural tube; us, 

 primitive segment. 



[ * MACBRIDB'S recent investigations (Nos. VIII. and VIII. a) show that the 

 mesoderm arises as a series of true gut-pouches. ED.] 



