MARINE TELEOSTEAN DEVELOPMENT. 91 



Organs of the Hypoblast or inner layer. 



Notochord or central axis. 



In the earliest section of the trunk no trace of the notochord 

 is visible, a single layer of hypoblast limiting the neurochord 

 below with the plates of the mesoblast above at the sides. 



About the time when the lip of the blastopore has reached 

 the equator, a median mass of rounded cells is observed between 

 the neurochord upon which they press and the thin median 

 stratum of hypoblast below — in the middle region of the trunk ; 

 it gradually extends forward, ending above the cardiac rudiment 

 on the second day after the blastopore closes. 



In the early condition of the notochord the cells forming 

 it are more numerous; thus from six to eight cells extend 

 across the diameter of the chord, due to irregular vertical septa 

 (vacuolation), after the breaking down of cells. Later they 

 form a meshwork, the outermost portions of which constitute a 

 limiting membrane with occasional nuclei. Intruding meso- 

 blast limits the chord below and later still surrounds it with a 

 very thin layer. This is the simple condition of the parts 

 about the 20th day after hatching — the sheath proper being 

 very thin, and the mesoblastic sheath (perichordal) but little 

 increased in thickness. From the latter the future vertebraj 

 are formed, and an external limiting layer can be made out. 

 This structure (external elastic membrane) gives origin to the 

 arches, the upper (neural) preceding the lower (haemal) in 

 development. 



In some forms (e.g. in the snake) cartilage develops preco- 

 ciously in the vertebral column, but in many cases the arches 

 and outer laminae of the vertebral bodies are not preceded by 

 preformed cartilage but by ossific matter of a clear, homo- 

 geneous, and brittle nature. Indeed in more advanced stages 

 a spicular sheath has been found in the connective tissue out- 

 side the external limiting membrane. 



