STRUCTURE AND DEVELOPMENT OE LEPIDOSTEUS. 
389 
different. At the front end of the anal fin the paired haemal arches suddenly enlarge 
and extend ventral wards (Plate 28, fig. 67, h.a.). 
Each succeeding pair of arches becomes larger than the one in front, and the two 
elements of each arch first nearly meet below the caudal vein (Plate 28, fig. 67) and 
finally actually do so, forming in this way a completely closed haemal canal. At the 
point where they first meet the permanent caudal fin commences, and here (Plate 28, 
fig. 68) we find that not only do the haemal arches meet and coalesce below the caudal 
vein, but they are actually produced into long spines supporting the fin rays of the 
caudal fin, which thus differs from the other fins in being supported by parts of the 
true vertebral column and not by independently formed elements of the skeleton. 
Each of the large caudal haemal arches, including the spine, forms a continous whole, 
and arises at an earlier period of larval life than any other part of the vertebral 
column. We noticed the first indications of the neural arches in the larva of about 
a week old, while they are converted into fully formed cartilage in the larva of three 
weeks. 
The neural and haemal arches, resting on the membrana elastica externa, do not at 
this early stage in the least constrict the notochord. They grow gradually more 
definite, till the larva is five or six weeks old and about 26 millims. in length, but other¬ 
wise for a long time undergo no important changes. During the same period, however, 
the true sheath of the notochord greatly increases in thickness, and the membrana 
elastica externa becomes more definite. So far it would be impossible to distinguish 
the development of the vertebral column of Lepidosteus from that of a Teleostean Fish. 
Of the stages immediately following we have unfortunately had no examples, but 
we have been fortunate enough to obtain some young specimens of Lepidosteus* which 
have enabled us to work out with tolerable completeness the remainder of the develop¬ 
mental history of the vertebral column. In the next oldest larva, of about 5 - 5 centims., 
the changes which have taken place are already sufficient to differentiate the vertebral 
column of Lepidosteus from that of a Teleostean, and to show how certain of the 
characteristic features of the adult take their origin. 
In the notochord the most important and striking change consists in the appearance 
of a series of very well marked vertebral constrictions opposite the insertions of the 
neural and haemal arches. The first constrictions of the notochord are thus, as in 
other Fishes, vertebral; and although, owing to the growth of the intervertebral 
cartilage, the vertebral constrictions are subsequently replaced by intervertebral con¬ 
strictions, yet at the same time the primitive occurrence of vertebral constrictions 
demonstrates that the vertebral column of Lepidosteus is a modification, of a type of 
vertebral column with biconcave vertebrse. 
The structure of the gelatinous body of the notochord has undergone no important 
change. The sheath, however, exhibits certain features which deserve careful descrip- 
* These specimens were given to us by Professor W. K. Parker, who received them from Professor 
Burt Gr. Wilder. 
