344 



EMBRYOLOGY. 



elongated plates (mk), which as in Amphioxus are arranged perpen- 

 dicularly to the surface of the chorda and neural tube. Since in 

 Petromyzon the primitive segments are destitute of cavities, the two 

 epithelial layers lie immediately in contact, and are continuous with 

 each other, both dorsally and ventrally, by means of transitional 

 cells (WZ), in the same way that in the fundament of the lens its 

 epithelium is continuous with the lens-fibres. Muscle-fibrillae (mf) 

 are now differentiated on both the broad surfaces of the cell-plates. 



Thus arise muscle-layers (Mus- 

 kelblatter) which are perpen- 

 dicular to the chorda. These 

 layers are each composed of 

 two sheets of the finest fibrillse, 

 running parallel to one an- 

 other. The two sheets are 

 separated from each other by 

 a delicate film of cementing 

 substance ; one of them owes its 

 existence to one formative cell, 

 the other to an adjacent cell. 

 In older larvae the primi- 

 tive segments spread out both 

 above and below ; accom- 

 panying this process there is 

 a continual formation of new 

 muscle-layers from the pre- 

 viously mentioned cells (WZ}. 

 The upper and lower margins 

 of the primitive segments 

 therefore constitute a zone 

 of proliferation, by means of 

 which the musculature of the 

 trunk is continually growing further dorsad and ventrad. 



At a later stage of development, in larvae six weeks old (fig. 191), 

 the muscle-layers are converted into Miiskelkdstchen (k), as SCHNEIDER 

 has named these peculiar definite structural elements of the Oyclo- 

 stomes. The facing fibrillge- sheets of two adjacent layers (Blatter; 

 unite with each other along their margins. Since these sheets have 

 been produced on the two sides of one cell-plate, each formative cell 

 is now surrounded on all sides, as though with a mantle, by the 

 fibrillae which it has generated. 



Ch 



Fig. 190. Cross section through the trunk-muscu- 

 lature of a larva of Petromyzon Planeri 14 

 days old. Magnified 500 diameters. 



N and Ch, the part of the cross section which is 

 adjacent to the neural tube and the chorda ; 

 cfa, skeletogenous sheath of the chorda ; ep, 

 epidermis ; ae, outer epithelial layer of the 

 primitive segment ; mk, nuclei of muscle-cells ; 

 mf, muscle-nbrillse in cross section ; WZ, zone 

 of growth transition from the outer cell-layer 

 to the muscle-forming layer of the primitive 

 segment. 



