780 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



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 millions, in length, but 

 otherwise for a long time undergo no important changes. Dur- 

 ing the same period, however, the true sheath of the notochord 

 greatly increases in thickness, and the membrana elastica ex- 

 terna becomes more definite. So far it would be impossible to 

 distinguish the development of the vertebral column of Lepidos- 

 teus 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 1 , which have enabled us to 

 work out with tolerable completeness the remainder of the de- 

 velopmental 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 

 Lcpidosteus from that of a Teleostean, and to shew 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 verte- 

 bral constrictions opposite the insertions of the neural and licenial 

 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 subse- 

 quently replaced by intervertebral constrictions, yet at the same 

 time the primitive occurrence of vertebral constrictions demon- 

 strates that the vertebral column of Lepidostens is a modification 

 of a type of vertebral column with biconcave vertebrae. 



1 These specimens were given to us by Professor W. K. Parker, who received 

 them from Professor Burt G. Wilder. 



