238 
ORIGINAL ARTICLES. 
rise, on the one hand, to a fibrous sheath, on the other, to four dif¬ 
ferent cartilaginous plates closely united with that sheath, so that 
each lateral half of a centrum primitively consists of two portions. 
Perhaps the investing mass of the notochord may develope in a 
similar manner in some other vertebrata; since primarily, before 
chondrification, it thickens on each side in two places, and then the 
two thickenings approximate and coalesce. 
§ 9. Another deviation from the ordinary mode of development is 
to be seen in the spinal column of certain tail-less Batrachia, namely 
of liana paradox a and JBufo obstetricans. In these the vertebral 
centres are developed, not from complete cartilaginous rings, but 
from half rings, which lie on the notochord and only partly inclose 
it, since they leave its under side uncovered. 
§ 10. A third deviation is presented by the spinal column of the 
Sharks. It is usual in Vertebrata to find the interspace between two 
adjacent vertebral arches filled only by fibrous membrane, the liga- 
mentum intercrurale. But in these animals, there is, in almost every 
such interspace, an additional oblong plate of cartilage, which re¬ 
sembles a neural arch in form and size. Similar but much smaller 
intercalary cartilages are seen in the Sturgeons. 
§ 11. Lastly, it is to be noted, that the nerves which the spinal 
chord gives off laterally, usually pass between the neural arches of the 
vertebrae, while, in the Sharks, they traverse the neural arches them¬ 
selves, so that each crus exhibits a foramen for the passage of a nerve. 
§ 12. The anterior, more or less pointed end of the notochord pene¬ 
trates into the skull, and contributes to form the lower wall of the 
future brain case. Probably, without any exception, it extends 
only to between the pair of vesicles (more or less open towards the 
cranial cavity and chondrifying very early), which belong at first to 
the lateral walls of the brain case, and developing into the skeletal 
parts of the auditory labyrinth—change their form, as they grow, 
but little in a few cases—while, for the most part, they become much 
altered. 
Within the lower wall of the brain case there now accumulates 
around the cephalic part of the notochord, and probably at first to 
the right and left of it, a coarsely granular blastema, forming two 
thin streaks, which, as it were, fringe the part of the notochord in 
question, and then become converted into cartilage. That such is 
the case is rendered probable by the structure of Aninioccetes 
branchialis, in which animal the development of the skull is at its 
lowest stage. The direct study of the development of various 
Vertebrata has, however, yielded no certain results upon this point; 
on the contrary, the cephalic part of the vertebral column, when 
such a blastema destined for the formation of the skull was depo¬ 
sited, was surrounded by it as by a sheath. 
The further morphological relations of this sheath, or of the in¬ 
vesting mass, are remarkable, and are different in different animals. 
