272 Leonard ^Y. Williams 



posterior part of the trunk by the cartilaginous vertebrae and the con- 

 stricted precartilaginous or blastemal intervertebral discs; and poste- 

 riorly by the constricted blastemal vertebrge and the dense blastemal 

 discs. 



The notochordal crests correspond, at first, quite closely with the 

 intervertebral discs; but later (assuming that the posterior portion of 

 the notochord represents an earlier, and the anterior part a later phase 

 of identical processes) the crests appear to be a little in front of the 

 centers of the discs. As the notochord is driven from the vertebrge, 

 it forces its sheath downward and outward so that its lower limit in 

 the intervertebral disc is brought down as far as the troughs of the 

 notochordal undulations (Fig. 10). The enlargement consequently 

 becomes irregularly fusiform, its lower surface being flat. As more 

 tissue is forced into the disc, the enlargement bulges downward sharply 

 at a point near the middle of the disc and somewhat behind the crest 

 of the notochordal wave. This process continues until, in an embryo 

 of 12 mm., the notochordal enlargements are roughly diamond-shaped 

 (Fig. 17). The lower angle is always less acute and prominent than 

 the upper or primary angle, which represents the crest of the notochordal 

 wave. The notochordal sheath is not broken by the expansion of the 

 notochord. The inner notochordal sheath appears late and is relatively 

 thin. The accumulation of mucin within the intervertebral enlarge- 

 ment of the notochord is quite large. The anterior end of the noto- 

 chord terminates, at a point midway between the hypophysial fossa and 

 the foramen magnum, in a rounded knob. The cranial portion of 

 the notochord, with the exception of the knob just mentioned, lies 

 upon the upper surface of the cartilage, under the perichondrium, and 

 forms a distinct ridge. The head of each rib is continuous with the 

 intervertebral disc, and the tubercle is continuous with the neural 

 process of the vertebra behind the disc. The transverse process (or 

 cervical rib) of each cervical vertebra is continuous with the neural 

 arch above and with the body of the verteljra at the base of the arch. 

 The head of the rib seems to be displaced forward in the trunk of the 

 opossum. 



The process of differentiation of the vertebral column of the opossum 

 seems to be identical with that in the pig, but the scantiness of the 

 material at hand does not permit a precise determination of the condi- 

 tions in the opossum. I am quite sure, however, that in both animals 

 there should be recognized four distinct processes of vertebral differ- 



