THE ORGANS OF THE MIDDLE GEEM-LAYER. 



349 



In tlie Selachians the skeletogenons layer, the origin of which has 

 already been described, grows upward at the side of the chorda (fig. 

 195 Vr). Outside of this layer on6 finds the part of the primitive 

 segment which serves for the formation of muscle. This consists of 

 an inner layer {mp') and an outer layer {mp), Tvhich are separated 

 from each other by the remnant of the cavity of the primitive segment 

 (fig. 194 h). The inner layer (fig. 195 mp') is in contact with the 

 skeletogenous tissue (Vr), and is composed of numerous, superposed, 

 spindle-shaped cells, which are arranged longitudinally and give rise 

 to transversely striped muscle-fibrillse ; they correspond to the inner 

 wall of the primitive segment in the larvse of Amphioxus (fig. 189) 

 and Oyclostomes, which Ls in direct contact with the chorda. The 

 outer layer lies in contact 



(jnp 



with 



Fig. 195. —Horizontal longitudinal section through the 

 * trunk of an embryo of ScyUium, after Balfour. 



The section is made at the height of the chorda, and 

 Bhowa the separation from the muscle-plates of the 

 cells which form the bodies of the vertebrae. 

 Chorda ; ep, epidermis ; Yr^ fundament of the 

 bodies of the vertebrae ; mp, outer cell-layer of 

 the primitive segment ; mp', portion of the primi- 

 tive segment which has already been differentiated 

 into longitudinal muscles (muscle-plate). 



cA, 



the epidermis, and 

 remains for a long time 

 composed of cubical epi- 

 thelial cells. Dorsally and 

 ventrally it bends around 

 into the muscle - forming 

 layer, and here contributes 

 to the enlargement of the 

 latter, as in Amphioxus 

 and the Oyclostomes, by 

 its cells becoming longer 

 and being metamorphosed 

 into muscle-fibres (fig. 185). 

 The muscle - plate then 

 spreads out farther into 



the wall of the trunk both above and below (figs. 185 and 205). 

 At the same time its cavity (myocoel) gradually disappears. The 

 muscle-forming layer (fig. 185 mp'') continues to increase in thickness, 

 since the number of muscle-fibres becomes greater ; the outer layer 

 also loses, rather late it is true, its epithelial character, and is con- 

 cerned on the one hand in the development of the corium (fig. 205 

 cp), while on the other it furnishes an additional outer, thin muscle- 

 lamella. This observation, made by Balfoue, has often been called 

 in question, but has recently been confirmed by van Wijhe. 



In Reptiles, Birds, and Mammals the proliferation of the primitive 

 segments which furnishes the skeletogenous tissue is still more 

 extensive than in Selachians. Thereby the muscle-plate, or the 

 dorsal plate, as it is also called, is crowded farther away from the 



