94 ON THE MORPHOLOGICAL CONSTITUTION OF 



in the lancelet, and envelopes the column formed by the 

 bodies of the vertebrae in other fishes. It then bounds the 

 neural and haemal cavities, and from these cavities passes in 

 the mesial plane above and below to the neural and haemal 

 margins of the body. Corresponding cartilaginous and osseous 

 parts are imbedded in these fibrous neural and haemal laminae. 

 From the right and left sides of this deep or central system of 

 fibrous laminae, other laminae extend outwards between the 

 myotomes, and are connected to the deep fibrous layer of the 

 integument. The bones usually distinguished as " additional 

 ribs," "upper ribs," "epipleural spines," "diverging appendages," 

 are imbedded in these metamyotomic laminae ; and as the class 

 of radiating bones to which these so-called additional ribs be- 

 long maybe conveniently distinguished as actinapophyses (axrts- 

 ftog), I apply the term actinal to the metamyotomic fibrous 

 laminae of the sclerome. As those dermal bones or plates which, 

 from their histological as well as their teleological characters, 

 certainly constitute elements of the sclerome, are formed in 

 the layer of the integument to which the actinal sclerous 

 laminae are attached, this integumentary fibrous structure 

 must be considered as constituting a dermal sclerous lamina, 

 and so completing the fibrous portion of the sclerome. 



The sclerome thus consists fundamentally of a fibrous 

 structure, which surrounds the " corda dorsalis," bounds the 

 neural and haemal cavities, forms a mesial septum above and 

 below, separates the myotomes from one another, and, under 

 the integument, envelopes the deeper parts. 



The Development of Cartilaginous and Bony Elements in 

 the Fibrous Sclerome. The immediate development of certain 

 bones from or in a fibrous matrix, and of others in cartilage 

 previously formed in it, has given rise, among other questions, 

 to one as to whether the former are to be included in the 

 vertebrate system of bones. Now, while I admit the import- 

 ance of the embryological and histological facts which the 



