258 Leonard W. Williams 



(Froriep), not in the posterior half of the sclerotome as in man 

 (Barcleen). 



Tliese structures are more clearly defined in still later embryos, 7.S 

 mm. (Fig. 1), and it is evident that the differentiation of the axial 

 structures is brought about as much by the spreading apart of the 

 mesenchymal cells as by their aggregation or condensation. There 

 are in 5^,760 cubic microns about 100 nuclei in the disc and 29 in the 

 light transverse zones. The enumeration of nuclei in the dense regions 

 is too difficult to yield reliable results,^ and the anT)roximate number 

 only can be given. Below the level of the notochord the tissue of the 

 disc is much less dense than at the level of the notochord. The dense 

 tissue of the discs does not extend much above the notochord, but the 

 discs are now united by a median cord of dense tissue, the perichordal 

 septum, which surrounds the notochord and forms a dense band below 

 it. The rounded intersegmental zones of looser tissue are deeply con- 

 stricted between the notochord above and perichordal septum below, 

 but the two lateral portions of each seem always to be connected at 

 least by a slender cord of loose tissue which passes under the crest of 

 the corresponding notochordal undulation. 



The notochordal sheath appears first in embryos of 7 mm. and is ap- 

 parently fully formed in embryos of 7.8 mm. It is an anhistic mem- 

 brane about 1 to 1.5 microns thick and it is faintly striated concen- 

 trically. The formation of this sheath, or of the inner sheath which 

 appears later, does not affect the proper walls of the notochordal cells 

 which can still be seen inside the sheath. 



The loosening up of the axial mesenchyma reaches its maximum in 

 embryos of 9 mm., for there are 20 nuclei in 5,760 cubic microns in 

 the light zones and 63 in the discs (Fig. 2). In embryos of 10 mm. 

 the mesenchyma is denser, and the loose tissue of the vertebrae has 50 

 nuclei in the same volume. The mesenchymal tissue above the noto- 

 chord has increased largely, for in the 5.5 mm. embryo the notochord 

 was separated from the spinal cord by a small plate of mesenchyma 

 scarcely thicker than the notochord; it is now separated by more than 

 thrice its diameter. The perichordal septum surrounds the notochord 

 and the vertebral anlages. The septum, nevertheless, is incomplete, 

 for, as stated above, a small isthmus imder the notochordal crest con- 

 nects the lateral masses of the vertebral anlage. 



The multiplication of the cells of the discs and of the perichordal 

 septum has been sufficiently rapid, up to this age, to maintain approxi- 



