LATER LARVAL STAGES. 567 



The muscle-plate is then connected merely at its dorsal 

 edge by means of a mesentery-like band to the wall of the proto- 

 vertebra and otherwise hangs quite freely into the cavity of the 

 same. Its chief points of attachment are to the dissepiments. The 

 inner wall of the sclerotome becomes applied to the chorda and the 

 medullary tube, and here forms the skeletoyenou* layer which yields 

 the outer sheath of the chorda (4) and the neural continuation of the 

 latter. The outer layer of the sclerotome becomes applied to the 

 inner side of the muscle-layer and forms the internal sheath or 

 fiiitr.ia-layer (3). The lateral trunk-muscle of Amphioxus is not 

 entirely surrounded by fascia, since this layer only develops on its 

 inner side. 



All these layers, derived through differentiation from the wall of 

 the proto- vertebra, shift ventrally, pressing in between the ectoderm 

 and the somatopleura. The cutis-layer in this way comes to lie in 

 the ventral middle line, where it yields the lining of the cavity of the 

 ventral .fin (/ J, and the dorsal fin-cavities (I) seem in the same way 

 to be lined by the cutis-layer. From the floors of these cavities, the 

 fin-rays grow up later. The skeletogenous and the muscular layers 

 also shift ventrally. 



The method of formation of the layers agrees in all essential points 

 with that of the Craniata. In this type, beneath the epidermis, lies 

 the cutis-layer (1) which, in Amphioxus, retains its simple epithelial 

 diameter throughout life. This is followed by the myocoele, which 

 also here persists throughout life. Then comes the muscle-layer (2) 

 and on the inner side of the latter lies the fascia-layer (3) ; in the 

 Craniata the fascia is also developed on the outer surface of the 

 muscles ; then comes the cavity of the sclerotome, and, finally, the 

 skeletogenous layer (4). The latter (as chorda- sheath) encloses the 

 diorda and also the medullary tube in the dorsal half of the body ; 

 in the lower half, it becomes applied to the somatopleure. The two 

 layers (skeletogenous layer and somatopleure) here form a delicate 

 partition- wall, the intercoelic membrane (o and 6} which separates 

 the cavity of the primitive vertebrae from the splanchnocoele. On 

 the inner side of the splanchnocoele lie the splanchnopleure and the 

 t-ntodermal intestinal epithelium. 



In the branchial region (Fig. 311) the condition of the body-cavity 

 is complicated through the development of the atrial cavity (/>), which 

 presses in between the splanchnocoele and the ventral part of the 

 myocoele. The latter then lies in the peribranchial fold and breaks 

 up into sections called by HATSCHEK (No. 8) the upper and lower 



