THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 595 



Immediately after its formation the chorda is in contact above 

 with the neural tube, below with the entoderm. and laterally 

 with the primitive segments. This relation is altered as soon as 

 the intermediate layer makes its appearance between the first 

 embryonic fundaments. Then a layer of cells grows around the 

 chorda (fig. 262), spreads itself out from here around the neural tube 

 above, and furnishes the foundation from which are developed the 

 segmented vertebral column and in front, in the region of the five 

 brain-vesicles, the cranial capsule ; it has therefore received the 

 name of membranous vertebral column and of membranous cranial 

 capsule (membranous primordial cranium) ; it is also appropriately 

 designated as skeletogenous layer, the envelope which invests the 

 chorda being called the skeletogenous sheath of the chorda. 

 (Compare p. 172 for an account of the first formation of it.) 



The mesenchyme also spreads out laterally in the embryo, pene- 

 trates into the spaces between primitive segments, and is. converted 

 into thin plates of connective tissue (ligamenta interinuscularia), by 

 means of which the musculature of the trunk is parted into separate 

 muscle segments (myomeres). The muscle-fibres find attachment 

 and support upon both the anterior and posterior faces of these 

 plates. 



Such a condition is permanently preserved in Amphioxus lanceo- 

 latus. The chorda, with its sheath, is the only firm skeletal structure. 

 Fibrous connective tissue (membranous vertebral column) envelops 

 it and the neural tube, and sends out into the musculature of the 

 trunk the intermuscular ligaments. 



When the originally membranous tissue surrounding the chorda 

 and neural tube is followed in its further development in the 

 embryos of the higher Vertebrates, it is to be seen that it succes- 

 sively undergoes two metamorphoses : that at first it is partially 

 chondrified, and that subsequently the cartilaginous pieces are 

 converted into osseous tissue ; or, in other words, the first-established 

 membranous vertebral column is soon converted into a cartilaginous, 

 and this in turn is replaced by a bony one, and in the same manner 

 the membranous primordial cranium is transformed into a cartila- 

 ginous, and this in turn into a bony cranial capsule. 



The three stages which succeed one another in the development 

 of the higher Vertebrates are also encountered in a comparative- 

 anatomical investigation of the axial skeleton in the series of 

 Vertebrates, and in such a manner that the condition, which in 

 many classes appears only as a transitory embryonic one, is i-etained 



