290 EMBBYOLOGY OF THE LOWEE VEETEBEATES ch. 



individual cells develop in their cytoplasm fluid vacuoles which 

 increase in size and become confluent until at last the cell takes the 

 form of a comparatively thin layer of protoplasm surrounding an 

 enormous vacuole and containing embedded in its substance at one 

 point the nucleus. The turgescent condition of the cells inflated 

 with fluid gives them the firmness which enables the notochord to 

 carry out its function as a supporting structure. 



The inflation of the cells with fluid carries with it another result 

 namely a great increase in size of the individual cells. This in turn 

 causes a great increase in size of the notochord as a whole, showing 

 itself particularly by increase in diameter but also by increase in 

 length. The latter is not able to take place with perfect freedom 

 and the result is that the individual cells tend to be compressed into 

 the form of transverse discs. 



The notochordal rudiment at an early stage becomes covered 

 by a thin, elastic, highly refracting, cuticular formation known as 

 the primary sheath of the notochord (" elastica externa"). After 

 the formation of this the superficial layer of the notochord soon 

 resumes its cuticle-forming activity but now in a somewhat modified 

 form — a secondary or " fibrous " sheath, thicker and more jelly-like 

 in appearance, being produced internal to the primary sheath. In 

 Cyclostomes and Sturgeons this secondary sheath remains through- 

 out life without conspicuous change beyond increase of thickness 

 and the assumption of a tough fibrous character and is physiologically 

 the most important part of the axial skeleton of the trunk region. 



The superficial layer of notochordal cells, lying in immediate 

 contact with the inner surface of the secondary sheath, do not as a 

 rule undergo the process of vacuolation which affects the inner cells. 

 They remain as a layer of compact protoplasmic cells known as the 

 notochordal epithelium. 



In some Vertebrates (Dipnoi, Agar, 1906) a short stretch of noto- 

 chord, from the tip backwards, degenerates within its primary sheath 

 at an early stage, breaking up into loose mesenchyme. As the 

 notochord behind the degenerated portion grows in length its front 

 end is pushed forwards so as to re-occupy the vacated portion of 

 primary sheath. The extent to which this process takes place 

 throughout the Vertebrata in general, and also its meaning, are 

 deserving of further enquiry. 



Hypochokd (Subnotochordal Eod). — In the anamniotic Verte- 

 brates there is formed what is apparently an accessory notochord 

 lying ventral to the true notochord and hence known as the Hypo- 

 chord or Subnotochordal rod. This organ (see Gibson, 1910) arises 

 after the notochord and in an entirely similar manner, i.e. as a 

 longitudinal rod of cells split off from the endoderm in the mid- 

 dorsal line and sometimes possessing a distinct groove along its 

 lower surface facing the enteric cavity. On its surface it normally 

 develops a primary sheath precisely like that of the notochord. 



We may be sure, from its wide distribution amongst the more 



