LATER LARVAL STAGES. 



565 



plates, the septa disappear and the segmental cavities enclosed 1>\ 

 them flow together to form a common cavity surrounding the intestine 

 (splanchnocoele). The disappearance of the ventral mesentery leads 

 to intercommunication between the right and left halves of the 

 splanchnocoele. 



The proto- vertebrae enclose segmentally-arranged cavities which, 

 however, are not quite symmetrical in relation to the median plane 

 (p. 545). These are the cavities of the proto- vertebrae or the 

 myocoeles (Fig. 153, 7). The walls of each pro to- vertebra consists of 

 a parietal (1) and an inner (2) layer. The parietal layer (1), which 

 consists of flat cells, applies itself closely to the ectoderm, and since 



FIG. 299. Transverse section 

 from the middle of the body 

 of an Amphioxus larva with 

 five branchial clefts (after 

 HATSCHEK). 



FIG. 300. Diagrammatic render- 

 ing of the same section. 



A, epidermis ;/>', medullary tube ; C, chorda; t\, inner chorda-sheath ; U, intestinal 

 epithelium ; E, sub-intestinal vessel. 1, cutis-layer ; #, muscle-layer (lateral 

 trunk-muscle) ; 3, sclera-layer ; 4, boundary cells of the proto-vertebra ; 5, somato- 

 pleure ; 6, splanchnopleure. /, myocoele ; //, splanchnocoele. 



it yields the cutis of the adult, is sometimes termed the cutis-layer. 

 The cells of the inner layer, at the sides of the chorda and of the 

 medullary tube, elongate transversely and longitudinally, as described 

 above (p. 548), and, as they give rise to the muscle-fibres of the 

 myotome, this part is spoken of as the muscle-plate. 



At first, each muscle-cell contains only one fibrilla, but as the 

 number of fibrillae increases later they are arranged in regular order 

 one above the other. Several such groups of fibrillae occur in each 

 muscle-cell. The nuclei of the muscle-cells lie on the outer side 

 of the muscle-layer that turns towards the myocoele. Towards the 

 ventral side, the muscle-layer is continued into a pavement-epithelium 



