AN INTRODUCTION TO ZOOLOGY 



visceral or splanchnic mesoderm. The cells of the remaining part, 

 namely, that bordering on the notochord and neural canal, enlarge 

 in the horizontal plane to form a comparatively thick plate to which 

 the term myotome is applied. It is sometimes convenient to 

 differentiate between the myotome and the other two portions, i.e. 

 the splanchnic and somatic mesoderm, and so for these we use the 

 inclusive term lateral plate. The enlargement of the myotome 

 reduces the ccelomic cavity in its vicinity to a small cleft, the 



nt. 



FIG. 130. Amphioxus, transverse sections. From Bourne, 

 after Hatschek. 



A, B, C, transverse sections of embryos of Amphioxus of different ages, illustrating the forma- 

 tion of the neural tube, notochord, and mesoblastic somites, ack., archenteron ; mes., meso- 

 blastic somites; nch., notochord; np., neural plate; nt., neural tube. D, transverse section 

 through an older embryo in which the mesoblastic somites are completely separated from the 

 archenteron. mp., muscle plate ; cce., ccelom ; en., mesenteron ; the other lettering as above. 

 E, diagrammatic transverse section through a larva with five gill-slits, myc., myoccele ; spl., 

 splanchnocoale. F, diagrammatic transverse section of a young Amphioxus shortly after the 

 metamorphosis ; the section is taken between the atriopore and anus. ao., aorta ; df., dorsal 

 fin; scl., sclerotome ; vf., ventral fin ; the other lettering as above. 



myoecel. On the other hand the remaining part of the ccelom, now 

 known as the splanchnoccel, enlarges, and, finally, the partitions 

 between the successive splanchnoccels break down, giving rise to 

 the one large ccelom of the adult. While the union of the ventrally 

 situated portions of the enteroccelic cavities is taking place, a series 

 of partitions arise that shut off the myoccels which do not run 

 together 



