694 



EMBRYOLOGY. 



below the neural tube and above the intestine and aorta. It reaches 

 from the front end of the base of the mid-brain to the end of the tail. 



For a time after its establishment the front end of the chorda remains in 

 union at a small place with the epithelium of the fore -gut. This place is 

 immediately behind the upper attachment of the primitive pharyngeal 

 membrane (Eachenhaut). There is here found, a little behind the hypo- 

 physial pocket, a slight depression in the epithelial lining of the fore-gut — 

 Sebssel's pocket or the palatal pocket of Selenka. It is only some time 

 after the rupture of the pharyngeal membrane that the chorda becomes 

 detached from the intestinal epithelium and ter- 

 minates free in the mesenchyma, often with a 

 hook-like end (Kbibel, Kann, Cakius). 



In the case of Amphioxus the chorda is 

 the only skeletal structure present in the 

 •whole of the soft body; in the lower Ver- 

 tebrates (Cyclostomes, Fishes, Amphibia) it 

 exists even in the adult animals as a more 

 or less important organ ; but in the Amniota 

 it is reduced almost to obliteration, and only 

 in the earliest stages of development plays 

 a role as the forerunner, as it were, of the 

 higher form of axial skeleton which finally 

 takes its place. While reference is made 

 to previous portions of the text-book for in- 

 formatioA about the first development of the 

 chorda, its further metamorphosis may be 

 treated of here more at length. This varies 

 according as the chorda becomes a really 

 functional organ or soon begins to degene- 

 rate. 

 In the first instance, when the band of chordal cells has been 

 constricted off from the inner germ-layer, it becomes more sharply 

 limited at its periphery by the secretion of a firm, homogeneous 

 envelope, the sheath of the chorda (fig. 324 cs). Then the cells 

 increase in size by the accumulation of fluid within their protoplasm, 

 which finally exists in the form of a thin superficial layer only ; the 

 cells become enveloped in firm membranes, thus acquiring exactly 

 the appearance of plant cells. But directly beneath the sheath of 

 the chorda (fig. 324) the cells remain small and protoplasmic and 

 constitute a special layer, the chordal epithelium, which by proli- 

 feration and metamorphosis of its elements causes an increase of the 

 substance of the chorda. 



Kg. 301 — Cross section 

 through the vertebral 

 column of a young 

 Salmon, after Gegen- 



BAUB. 



cs, Sheath of the chorda ; 

 k, neural arch ; ff, 

 haemal arch ; m, spinal 

 cord ; a, dorsal aorta ; 

 V, cardinal veins. 



