INTRODUCTION 



the tissue which gives rise to many of the dermal (membrane) bones 

 and other skeletal bodies near the surface of the animal. 



With further growth, the mesenchyme from all sources invades 

 all spaces between other structures, forming a continuum which 

 has been called the membranous skeleton, but as this is illy defined, 

 and as a large part of it never becomes truly skeletal, it needs no 

 further mention, save to say that all true skeletal parts — cartilage 

 and bone — are formed within its limits, and that wherever it occurs 

 there is the potentiality of skeletal formation. 



More definitely, the chief locations of skeletogenous tissue in the 

 trunk are the following (fig. 3) ; Between the ectoderm (epidermis) 



Fig. 3.^-Location of skeletogenous tissue in caudal region, a, caudal artery; d, 

 cerium; e, epaxial muscles; h, horizontal septum; hy , hypaxial muscles; i, intersection of 

 {m) myosepta with horizontal septum; ms, median septum; n, notochord; s, spinal cord; 

 V, caudal vein. 



and the trunk muscles is the corium. This is connected at the mid- 

 dle line, above and below, by a vertical median septum of mesen- 

 chyme extending between dorsal and ventral sides of the trunk, 

 passing on either side of the spinal cord, notochord, dorsal aorta, 

 and the chief viscera located in the body cavity. The muscles of 

 either side of the embryo (also of adult fishes) are segmented, the 

 segments (myotomes) being separated by vertical partitions of mes- 

 enchyme (myosepta) extending from corium to the median septum. 

 The myotomes of either side are divided into dorsal (epaxial) and 

 ventral (h3^axial) muscles by a similar horizontal septum, also 



