Develojjment uf the Food-Fishes. 453 



less ; but the appearance of a central lumen is accompanied 

 by faint irregular pulsations. It is a simple tubular struc- 

 ture, and its cellular transparent walls assume a rugous 

 appearance. Though no haemal fluid can be detected until 

 some days after emergence, the cardiac contractions were first 

 noted on the following days : — G. morrhua, sixth day ; G. 

 merlangus^ eighth day ; P. limanda, tenth day ; G. ceglefinus^ 

 eleventh day ; and T. gurnardus on the fourteenth day. The 

 rhythmic movements when commencing are very slow; but 

 when they become regular they average from twenty to thirty 

 pulsations per minute, and when the heart is in full vigour 

 the rate increases to forty-five beats per minute, while the 

 embryo is still within the ovum. 



As early as the fourth or fifth day a primitive coelomic 

 cavity develops as a horizontal fissure traversing eacli of the 

 lateral muscle-plates and dividing the somatopleuric from the 

 splanchnopleuric portions. In the proximal niche of the 

 cavity thus formed upon each side of the embryonic axis a 

 special part is differentiated which performs the function of 

 an excretory duct *. At a very early stage these longitudinal 

 tubes, whose walls consist of a single layer of cubical epi- 

 thelial cells, can be traced along the line where the somato- 

 pleure and splanchnoplcure remain continuous, each termi- 

 nating anteriorly in a crozier-shaped loop with an infundibular 

 opening, near which is a plicated body enclosed in a capsule, 

 apparently a single glomerulus. Shifting their position, these 

 tubes, tlie Wolffian ducts, lie on the ventral side of, and pas3 

 parallel to, tlie vena vertehralis, and posteriorly tliev unite to 

 form a large urinary vesicle immediately below the origin of 

 the tail, prominently seen in the newly-emerged embryo. 



Notochord and Vertebral Arches. 



The notochord arises as a rod of small cells almost undis- 

 tinguishable from the neurochordal and mesoblast cells ; but 

 transverse cavities soon appear, produced by the breaking up 

 of the original cells, which again give place to more spacious 

 chambers filled with juicy protoplasm, much vacuolated, and 

 forming a reticulated meshwork of great complexity. Though 

 flexible, the notochord possesses some rigidity, due to the 

 pressure of the protoplasmic contents of the metamorphosed 

 cells and the continuity of their membranous walls. From 

 the outermost cells a cuticular sheath is formed, but it is thin 

 and shows little lamination. Outside the chordal sheath a 



• Vide Sedgwick, " Developmeut of the Kidney," Quart. Joura. Micr. 

 Sci. vol. xxiv. p. 64. 



Ann, d: Mag. N. Hist. Ser. 5. Vol. xvii. 31 



