ORIGIN OF THE GERMINAL LAYERS. 



291 



some of the elements subsequently come from the epiblast ; and in Cepha- 

 lopoda it begins as a ring of cells round the edge of the blastoderm. 



Polyzoa Entoprocta. — It originates from a pair of cells at the lips of 

 the blastopore. 



Chaetopoda — Euaxes. It arises as a ridge of cells at the lips of the 

 blastopore (fig. 200). 



Gephjrrea — Bonellia. It arises (fig. 206) as an infolding of the epi- 

 blastic lips of the blastopore. 



Nematelminthes — CucuUanus. It grows backwards from the hypo- 

 blast cells at the persistent oral opening of the blastopore. 



Tracheata — Insecta. It grows inwards from the lips of the germinal 

 groove (fig. 207), which probably represent the remains of a blastopore. Part 

 of the mesoblast is probably also derived from the yolk- cells. A similar 

 though more modified development of the mesoblast occurs in the Araueina 

 (fig. 214). 



Crustacea — Decapoda. It partly grows in from the hypoblastic lips 

 of the blastopore, and is partly derived from the yolk-cells (fig. 208). 



Fig. 209. Thkee stages in the development of Sagitta. (A. and C. after 



Biitschli, and B. after Kowalevsky.) 



The three embryos are represented in the same positions. 



A. Eepresents the gastrula stage. 



B. Bepresents a succeeding stage, in which the primitive archenteron is com- 

 mencing to be divided into three. 



C. Eepresents a later stage, in which the mouth involution (m) has become con- 

 tinuous with the alimentary tract, and the blastopore has become closed, 



m. mouth; al. alimentary canal; ae. archenteron; hl.p, blastopore; pv. peri- 

 visceral cavity ; sp. splanchnic mesoblast ; so, somatic mesoblast ; ge. generative 

 organs. 



2. The mesoblast is developed from the walls of hollow out- 

 growths of the archenteron, the cavities of which become the body 

 cavity. 



Brachiopoda — The walls of a pair of outgrowths form the whole of 

 the mesoblast. 



Chsetognatha. — The mesoblast arises in the same manner as in the 

 Brachiopoda (fig. 209). 



EcMnodermata — The lining of the peritoneal cavity is developed 

 from the walls of outgrowths of the archenteron, but the greater part of 

 the mesoblast is derived from the amoeboid cells budded oflf from the walls 

 of the archenteron (fig. 210). 



19—2 



