34 



PHYLUM CEPHALOCHORDA. 



to the somites of the first pair. They are the " collar " somites 

 of MacBride (Fig. 19, 6'). The posterior outgrowths constitute 

 the coelomic grooves of MacBride. They remain open to the 

 gut for a considerable time behind, but as growth progresses 

 they are continually constricting off sacs anteriorly (Fig. 19, 5). 

 Eventually when about fourteen pairs of somites have been 

 formed they become separate from the endoderm of the arch- 

 enteron and form a solid plate on each side from which the 

 remainder of the somites are successively developed. In addition 

 to these two pairs of archenteric outgrowths there is a median 

 anterior outgrowth (Fig. 19, /). This grows back on each side 



FIG. 17. Transverse section through two embryos of AmpMoxus to show the enclosure of 

 the medullary plate and the formation of the coelcmic pouches. A section through an em- 

 bryo with the rudiment of one pcuch and of the notcchord. section through a slightly 

 older embryo, showing the complete separation of a coelomic poueh from the archenteroii 

 (from Korschelt and Heider alter Batscfcek) ; ok ectodeim ; ch rudiment of r.otochord ; 

 hb lateral ectoderm growing over the medullary plate ; ik, U endoderm*; Ih entejon 

 Ik future coelcm ; ntk coelcmic rcuch (future scmite) ; np medullary plate. 



and becomes separated from the gut as a single cavity with two 

 backwardly projecting horns. This unpaired anterior sac gives 

 rise to the head cavities ; it becomes divided into two, of which 

 the right shifts ventrally, becomes thin-walled, and forms the 

 cavity of the snout in the larva (Fig. 20). It becomes largely 

 obliterated in the adult. The left head-cavity, on the other 

 hand, becomes transversely placed beneath the notochord and 

 opens to the exterior on the left side in front of the mouth 

 (Fig. 20, w). It becomes the preoral (Hatschek's) pit, and gives 

 rise by the extension of its lips to the wheel organ (p. 20). The 

 collar-somites (so-called first pair) and the somites developed 

 from the coelomic groove give rise to the mesoderm, bo 



