120 DEVELOPMENT OF AMPHIOXUS. 



against the ectoderm. Later the dorsal margins of this 

 half-canal meet and fuse in the middle line, and so 

 produce the medullary tube* (Fig. 66). 



Origin of Mesodenn and Cwlont. 



In consequence of the flattening and incurving of the 

 medullary plate, pressure is brought to bear on the 

 dorsal wall of the archenteron, and the dorso-lateral bor- 

 ders of the latter acquire the form of two longitudinal 

 grooves (Figs. 65 A and B). It is from these grooves that 

 the archenteric pouches are split off. The grooves deepen, 

 and in doing so become divided up into a series of 

 pouches. Eventually the pouches become shut off from 

 the archenteron gradually from before backwards, and 

 then appear as closed cavities on either side of the 

 notochord, which has, in the meantime, been developing 

 (Fig. 6s F). 



In the higher Vertebrates the mesoderm arises as two 

 solid, lateral, longitudinal bands, which are split off from 

 the primitive endoderm. These mesodermic bands are at 

 first unsegmented, and might be taken to correspond with 

 the longitudinal grooves of the archenteron of Amphioxus, 

 as described above. Later, only the dorsal portion of the 

 mesodermic bands undergoes segmentation, while the 

 ventral portion, which becomes hollowed out to form 

 the general body-cavity, is never segmented in the crani- 

 ate Vertebrates. (Cf. Fig. 33.) In Amphioxus the whole 

 of the mesoderm is contained in the archenteric pouches, 

 and is, therefore, at first entirely segmented. 



As soon as the pouches have lost their primitive con- 



* In the Ascidian embryo the formation of the medullary tube takes place 

 after the manner typical of craniate Vertebrates (see below, IV.). 



