COMPARISON OF THE GERMINAL LAYERS. 



24., 



The formation of the Mesoblast and of the Notochord. 



Amphioxus. The mesoblast originates in Amphioxus, as in several 

 primitive invertebrate types, from a pair of lateral diverticula, con- 

 stricted off from the archenteron (fig. 180). Their formation com- 

 mences at the front end of the body and is thence carried backwards, 

 and each diverticuhim contains a prolongation of the cavity of the 

 archenteron. After their separation from the archenteron the dorsal 



FIG. 180. SECTIONS OF AX AMPHIOXUS EMBRYO AT THREE STAGES. (After Kowalevsky.) 



A. Section at gastrula stage. 



B. Section of an embryo slightly younger than that represented in fig. Ifi9 D. 



C. Section through the anterior part of an embryo at the stage represented in 

 fig. 169 E. 



up. neural plate ; HC. neural canal; mex. archenteron in A and B, and mesenterou 

 in C ; cli. notochord ; so. niesoblastic somite. 



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 so- 

 matic mesoblast. 



The notochord would almost appear 

 to arise as a third median and dorsal 

 diverticulum of the archenteron (fig. 180 

 c//). 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 (1) to 

 a general simplification of the organiza- 

 tion, and (2) to the non-continuation of blast of tail. 

 the notochord into the trunk. 



n.e 



FIG. LSI. TRANSVERSE OPTICAL 



SECTION OF THE TAIL OF AN Eil- 



BRTO OF PHALLUSIA MAMMILLATA. 

 (After Kowalevsky.) 



The section is from an embryo 

 of the same age as fig. 8 iv. 



ch. notochord ; n.c. neural 

 canal; me. mesoblast; al'. hypo- 



102 



