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MESSRS. F. M. BALFOUR ART) W. H. PARKER OR THE 
attempts to prove (l) that the so-called membrana elastica externa of the Teleostei is 
not a homogeneous elastica, but is formed of cells, and (2) that in the vertebral regions 
ossification first occurs in it. 
In Lepidosteus we have met with no indication that the membrana elastica externa 
is composed of cells; though it is fair to Gotte to state that we have not examined 
such isolated portions of it as he states are necessary in order to make out its struc¬ 
ture. But further than this we have satisfied ourselves that during the earlier stage 
of ossification this membrane is not ossified, and indeed in part becomes absorbed in 
proximity to the intervertebral cartilages; and Gegenbaur met with no ossification 
of this membrane in the later stage described by him. 
Summary of the development of the vertebral column and ribs. 
A mesoblastic investment is early formed round the notochord, which is produced 
into two dorsal and two ventral ridges, the former uniting above the neural canal. 
Around the cuticular sheath of the notochord an elastic membrane, the membrana 
elastica externa, is next developed. The neural ridges become enlarged at each inter¬ 
muscular septum, and these enlargements soon become converted into cartilage, thus 
forming a series of neural processes riding on the membrana elastica externa, and 
extending about two-thirds of the way up the sides of the neural canal. The haemal 
processes arise simultaneously with, and in the same manner as, the neural. They are 
small in the trunk, but at the front end of the anal fin they suddenly enlarge and 
extend ventralwards. Each succeeding pah* of hsemal arches becomes larger than the 
one in front, each arch finally meeting its fellow^ below the caudal vein, thus forming 
a completely closed hsemal canal. These arches are moreover produced into long 
spines supporting the fin-rays of the caudal fin, which thus differs from the other im¬ 
paired fins in being supported by parts of the vertebral column, and not by separately 
formed skeletal elements. 
In the next stage which we have had the opportunity of studying (larva of 
centims.), a series of very well-marked vertebral constrictions are to be seen in the 
notochord. The sheath is now much thicker in the vertebral than in the intervertebral 
regions : this is due to a special differentiation of a superficial part of the sheath, 
which appears more granular than the remainder. This granular part of the sheath 
thus forms a cylinder in each vertebral region. Between it and the gelatinous tissue 
of the notochord there remains a thin unmodified portion of the sheath, which is con¬ 
tinuous with the intervertebral parts of the sheath. The neural and hsemal arches are 
seen to be continuous with a cartilaginous tube embracing the intervertebral regions 
of the notochord, and continuous from one vertebra to the next. A delicate layer of 
bone, developed in the perichondrium, invests the cartilaginous neural arches, and this 
bone grows upwards so as to unite above with the osseous investment of separately 
developed bars of cartilage, which are directed obliquely backwards. These bars, or 
