i HI: <i NIK 



..I:I;.M \..\\ r.u. 



In the ^i-laciiian.N tin- keletogenous lay. of which has 



already IM en de>cnl d. rows upward at (lie >ide of (he chorda (fig. 



i'/-). ouiMdeof this layer one findfl tin- part of the primitive 

 -fLMiicnt which serves for the formation of nui-.-ie. This consists of 

 an innt r laser (/////) and an outer layer (mp) t which are separated 

 fnmi each ether by the remnant of the cavity of the primitive segment 

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

 >keletoi, r enous tissue ( FV), and is composed of numerous, superposed, 

 Bpindle-shaped cells which are arranged longitudinally and give rise 

 to transveis ly striped muscle-fibrillae ; 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 

 vent rally it bends around 

 into the muscle - forming 

 layer, and here contributes 

 to the enlargement of the 

 latter, as in Amphioxus 

 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 (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 

 al.-o 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 \Vi.nn:. 



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 



Fig. 195. Horizontal longitudinal section through the 

 trunk of an embryo of Scyllium, after BALFOUR. 



The section is undo 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 ; Kr, 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-plats). 



