304 CH^TOGNATHA. 



situated at the pole of the embryo opposite that at which the 

 mouth subsequently appears (fig. 164 A). The simple archenteron 

 soon becomes anteriorly divided into three lobes, which communicate 

 freely with the still single cavity behind (fig. 164 B). The two 

 lateral lobes are destined to form the body cavity, and the median 

 lobe the alimentary tract of the adult. An invagination soon arises 

 at the opposite pole of the embryo to the blastopore and forms the 

 mouth and oesophagus (fig. 164 B and C, m). 



At the gastrula stage there is formed a paired mass destined to 

 give rise to the generative organs. It arises as a prominence of six 

 cells, projecting from the hypoblast at the anterior pole of the archen- 

 teron, and soon separates itself as a mass, or probably a pair of masses, 

 lying freely in the cavity of the archenteron (fig. 164 A, ge). When 

 the folding of the primitive cavity takes place the generative rudi- 

 ment is situated at the hind end of the median lobe of the archenteron 

 in the position represented in fig. 164 C, ge. 



An elongation of the posterior end of the embryo now takes 

 place, and the embryo becomes coiled up in the egg, and when 

 eventually hatched sufficiently resembles the adult to be recognisable 

 as a young Sagitta. 



Before hatching takes place various important changes become 

 manifest. The blastopore disappears after being carried to the ventral 

 surface. The middle section of the trilobed region of the archenteron be- 

 comes separated from the unpaired posterior part, and forms a tube, blind 

 behind, but opening in front by the mouth (fig. 165 A, al). It constitutes 

 the permanent alimentary tract, and is formed of a pharyngeal epiblastic 

 invagination, and a posterior hypoblastic section derived from the primi- 

 tive archenteron. The anus is apparently not formed till compara- 

 tively late. After the isolation of the alimentary tract the remainder 

 of the archenteron is formed of two cavities in front, which open 

 freely into a single cavity behind (fig. 165 A). The whole of it con- 

 stitutes the ~body cavity and its walls the mesoblast. The anterior 

 paired part becomes partitioned off into a head section and a trunk 

 section (fig. 165 A and B). The former constitutes a pair of distinct 

 cavities (c.pv) in the head, and the latter two cavities opening freely into 

 the unpaired portion behind. At the junction of the paired cavities 

 with the unpaired cavity are situated the generative organs (ge). 

 The inner wall of each of the paired cavities forms the splanchno- 

 pleuric mesoblast, and the outer wall of the whole the somatic meso- 

 blast. The inner walls of the posterior cavities unite above and 

 below the alimentary tract, and form the dorsal and ventral mesen- 

 teries, which divide the body cavity into two compartments in the 

 adult. Before the hatching of the embryo takes place this mesentery 

 is continued backwards so as to divide the primitively unpaired 

 caudal part of the body cavity in the same way. 



From the somatic mesoblast of the trunk is derived the single 



O 



layer of longitudinal muscles of Sagitta, and part of the epithe- 

 lioid lining of the body cavity. The anterior termination of the 



