THE ORGANS OF THE MIDDLE GERM-LAYER. 



349 



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

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

 195 Fr). Outside of this layer one finds the part of the primitive 

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

 an inner layer (nip') and an outer layer (mp), which 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 (Fr), and is composed of numerous, superposed, 

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

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

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

 and Cyclostomes, which is in direct contact with the chorda. The 

 outer layer lies in contact 



with 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 



Fig. 195. Horizontal longitudinal section through the 

 trunk of an embryo of Scyllium, after BALFOUR. 



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

 shows the separation from the muscle-plates of the 

 cells which form the bodies of the vertebrae. 



cA, Chorda ; ep, epidermis ; Vr, 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). 



and the Cyclostomes, 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 (myoccel) 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 BALFOUR, 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 



