COMPARISON OF THE GERMINAL LAYERS. 293 



diverticula, constricted off from the archenteron (fig. 180). Their 

 formation commences at the front end of the body and is thence 

 carried backwards, and each diverticulum contains a prolongation 

 of the cavity of the archenteron. After their separation from the 

 archenteron the dorsal parts of these diverticula become divided by 

 transverse septa into successive somites, the cavities of which 

 eventually disappear ; while the walls become mainly converted 

 into the muscle-plates, but also into the tissue around the 

 notochord which corresponds with the vertebral tissue of the 

 higher Chordata. 



The ventral part of each diverticulum, which is prolonged 

 so as to meet its fellow in the middle ventral line, does not 

 become divided into somites, but contains a continuous cavity, 

 which becomes the body cavity of the adult. The inner layer of 

 this part forms the splanchnic mesoblast, and the outer layer the 

 somatic mesoblast. 



The notochord would almost appear to arise as a third 

 median and dorsal diverticulum of the archenteron (fig. 180 ch). 

 At any rate it arises as a central fold 

 of the wall of this cavity, which is 

 gradually constricted off from before 

 backwards. 



Urochorda. In simple Ascidians 

 the above processes undergo a slight 

 modification, which is mainly due (i) 



to a general simplification of the 



. FIG. 181. TRANSVERSE OPTI- 



organization, and (2) to the non- CAL SECTION OF THE TAIL OF AN 

 continuation of the notochord into 



the trunk. The section is from an embryo 



The Whole dorsal wall of the of the same age as fig. 8 IV. 



posterior part of the archenteron is can 



converted into the notochord (fig. blast 

 181 ch}, and the lateral walls into the mesoblast (me) ; so that 

 the original lumen of the posterior part of the archenteron ceases 

 to be bounded by hypoblast cells, and disappears as such. 

 Part of the ventral wall remains as a solid cord of cells (at) 

 The anterior part of the archenteron in front of the notochord 

 passes wholly into the permanent alimentary tract. 



The derivation of the mesoblast from the lateral walls of the 



