546 CEPHALOCHORDA. 



yields the mesodermal structures in the anterior region of the body. 

 It has recently been regarded by HATSCHEK (No. 8) as the rudiment, 

 of an independent pair of primitive segments, in which case, the 

 segment hitherto called the first would actually be the second. 



The walls which separate the consecutive segments, the so-called 

 dissepiments, at first run straight from the dorsal to the ventral side 

 (Fig. 285, A). Later, they curve backward (Fig. 286) and finally 

 they develop the characteristic angulation at the level of the upper 

 part of the chorda dorsalis (Figs. 287, 288). 



eo en 



FIG. 286. Stage in the development of Amphioxus in which there are iiiue primitive 

 segments (after HATSCHEK). In the fifth, sixth and seventh primitive segments, 

 the muscle formative cells (mz) are clearly marked, dv, anterior eutoderm-diverti- 

 culum ; ec, ectoderm ; en, entoderm ; m, cephalic process of the first primitive 

 segment ; mf, mesoderm-folds ; mp, pole-cells of the mesoderm ; mz, muscle 

 formative cells ; up, neuropore ; us', first primitive segment ; us", second primitive 

 segment. 



The primitive segments originally lie dorsally above the alimentary 

 canal (Fig. 283, mk). Later, they extend ventrally and thus grow 

 round the alimentary canal (Fig. 284). Those parts of the coelom 

 also which surrounded the canal are originally distinct from one 

 another, but at the stage in which the mouth forms, the ventral part 

 of the dissepiments degenerates, and consequently the originally 

 distinct coelomic cavities run together and form the splanchnocoele, 

 which, enclosed by the lateral plates, runs through the whole length of 

 the body as a continuous cavity. The right half of the splanchno- 

 coele is in open communication with the left half below the alimentary 

 canal. According to HATSCHEK, previous to this union of the ventral 

 part of the primitive segments that grow down on either side, a 

 -simple ventral lamella is formed which extends between the ectoderm 

 and the alimentary canal. In this ventral lamella the rudiment of 

 the first recognisable blood-vessel, the sub-intestinal vessel appears, 

 a vessel which lies later on the ventral side of the alimentary canal 

 covered by the splanchnopleure. The first rudiment of this vessel 



