CH^TOGNATHA. 367 



great value for the more general problems of embryology. The 

 development commences after the eggs are laid. The segmen- 

 tation is uniform, and a blastosphere, formed of a single layer of 

 columnar cells, is the product of it. An invagination takes 

 place, the opening of which narrows to a blastopore 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 archenteron, and soon separates itself as a mass, or probably 

 a pair of masses, lying freely in the cavity of the archenteron 

 (fig. 164 A. y ge). When the folding of the primitive cavity takes 

 place the generative rudiment 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 recog- 

 nisable 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 becomes 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 perma- 

 nent alimentary tract, and is formed of a pharyngeal epiblastic 

 invagination, and a posterior hypoblastic section derived from 

 the primitive archenteron. The anus is apparently not formed 

 till comparatively 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 constitutes the body cavity and its walls 



