No. 2.] MORPHOLOGY OF THE MYXINOIDEI. 193 



development of cartilage within the notochord, but not so 

 marked as in the anterior region. 



The skeletogenoiis layer (Plate XXII, Figs. 1-4, sk) lies 

 immediately external to the elastica extertta. It surrounds the 

 notochord, and above, on each side, it divides. One layer con- 

 tinues in contact with the elastica externay the other rises and 

 arches to form the neural tube. This tube forms a continuous 

 semicylindrical canal just above the notochord, with lateral 

 foramina for the exit of the spinal nerves. Within this neural 

 tube there is also a fibrous layer closely investing the spinal 

 cord (sp) and another which covers the so-called "fatty tissue" 

 {F) which more or less completely fills the dorsal part of the 

 tube. The skeletogenous layer is composed of strong fibrous 

 connective tissue, well supplied with blood vessels and nerves. 

 It is directly continuous with the intermuscular septa, as 

 shown in Figs, i and 2, inis. Above, it is continuous with 

 the median septum, separating the myotomes of the right and 

 left halves of the body (inds). Below, on each side, it is con- 

 tinuous with the fascia superficialis interna, which surrounds 

 and supports the body cavity. In the skeletogenous layer, 

 especially near the inferior lateral angles of the neural tube, 

 we occasionally find incipient patches of cartilage which are 

 interesting as the first traces of the cartilaginous structures 

 more fully developed in the head and tail region. 



Skeleton of the Head Region. 



(PI. XXII, Figs. 5,. 6; PI. XXIII, Fig. 7.) 



The skull, as a whole, may be described as forming two 

 tubes, a smaller dorsal and a larger ventral. The smaller 

 dorsal tube includes the cranium, posteriorly, and the nasal 

 tube, anteriorly. The larger ventral tube is composed of a 

 framework of cartilages surrounding the mouth and pharynx. 

 The mouth is surrounded by a system of cartilages supporting 

 the tentacles. For some distance behind the mouth the side 

 walls of the ventral tube are unsupported by cartilage, but pos- 

 teriorly the tube is surrounded by the complicated framework 



