352 DEVELOPMENT OF THE MESOBLAST. 



from a pair of cells at the lips of the blastopore, though very probably some 

 of the elements subsequently come from the epiblast ; and in Cephalopoda 

 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. 



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

 blastopore (fig. 200). 



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

 blastic lips of the blastopore. 



Nematelminthes. Cucullanus. It grows backwards from the hypoblast 

 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 Araneina 

 (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. THREE 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. Represents the gastrula stage. 



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

 mencing to be divided into three. 



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

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



m. mouth; al. alimentary canal; ae. archenteron; bl.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 

 outgrowths of the archenteron, the cavities of which become 

 the body cavity. 



