EATHKE ON THE DEVELOPMENT OF THE CBANIITM. 
235 
sheath. However this may be, the substance of this blastema, which 
I term the Investing mass of the notochord , very soon increases, chiefly 
in thickness, on each side; the thickening not being alike in all 
parts, but in some places thicker and in others thinner. This takes 
place in such a manner, that, on each side, a multitude of small plates 
are formed, of which each pair have a small, thinner, interspace. By 
degrees the plates grow towards one another above and below, and at 
last each pair coalesce into a ring. Consequently after a certain time 
the notochord is seen to be inclosed within a number of such rings, 
which lie in a series one between the other. # 
But before the plates thus coalesce into rings, the investing 
mass of the notochord, whence they are developed, grows upwards on 
the two sides of the future spinal cord, within the dorsal laminae, and 
here also attains, in one spot a greater, in another a less, thickness. 
These thicker and thinner spots correspond with the similar parts of 
the investing mass on the two sides of the notochord, or rather are 
to be regarded as the prolongations of them. 
Thus, after a certain time, there arises an appearance as if the 
just mentioned and already completed rings which inclose the 
notochord, had sent up rays to embrace the spinal cord on both 
sides. Later still, each pair of these rays come into contact above 
the spinal cord and coalesce, so as to form arches, which are borne 
by the rings inclosing the notochord. In the tails of osseous fishes, the 
same process goes on towards the opposite side, but the arches 
thus formed inclose, not parts of the nervous system, but the 
caudal arteries and veins. In the trunk of osseous fishes also, the 
rings which indicate the bodies of the vertebrae throw out down¬ 
wardly directed processes and, in certain fishes, the hindermost of 
these also coalesce so as to form arches; ordinarily, however, they re¬ 
main separate from one another, but mark themselves off from the 
bodies of the vertebrae and then appear as ribs. As in Fishes, so in 
other Yertebrata, processes are developed from the investing mass of the 
notochord in the trunk, and serve to inclose the subjacent viscera 
more or less completely. Some of them become distinct, either close 
to the rings, from which the bodies of the vertebrae will be formed, 
or at some distance from them, and are either developed into ribs only, 
or into ribs and transverse processes; others do not become distinct, 
and acquire the character of simple transverse processes. The 
transverse processes which are formed in many caudal vertebrae 
of different Amphibia and Birds have a similar origin. 
It is to be remarked that in some Amphibia, below these trans¬ 
verse processes, additional paired processes are formed, which in 
the Crocodiles coalesce into arches, but in the Snakes remain 
* Whether in many or in most Vertebrata two series of such plates, an upper 
and a lower (which however soon coalesce), are developed on each side, as is said by 
Von Baer to be the case in Cyprinus blicca requires further investigation. It is 
certainly not the case in the Snake. 
