TUBULATION OF ORGAN-FORMING AREAS IN AMPHIOXUS 



503 



5. TUBULATION OF THE MESODERM 



Tubulation of the mesoderm and the formation of a continuous antero- 

 posterior coelom in Amphioxus differs considerably from that found in the 

 subphylum Vertebrata. This fact becomes evident in tracing the history of 

 the mesoderm from the time of its segregation from the entoderm of the late 



DORSAL D I V E R T I C UlA)M^«B=ffl^ffl-H''l°W°M--l-r|-|-'i; 



Fig. 249. Various stages of development of Amphioxus. (A from Kellicott, '13, and 

 Conklin, '32; B from Kellicott, '13, slightly modified; C-I, slightly modified from Conklin, 

 '32.) (A) Six-somite stage, comparable to fig. 247G and H. The animal hatches about 

 the time that two pairs of somites are present. (B) Nine-somite stage. The larva at this 

 stage swims by means of cilia which clothe the entire ectodermal surface. (C) About 

 fourteen pairs of somites are present at this stage. Neurenteric canal is still patent. (D) 

 About 16 to 18 pairs of somites. Neurenteric canal is degenerating; mouth is formed. 

 (E) About 20 to 22 pairs of somites. Anal opening is established between this stage and 

 that shown in (D). (F) Trr.nsverse section, showing oral opening, looking from anterior 

 end of animal. (G) Same through anal area. (H) Frontal section of a 24-hour larva 

 near dorsal side showing notochord, somites (S-1, S-8, etc.) and undifferentiated tissue 

 at caudal end. Neural tube shown at anterior end. Nine pairs of somites are present. (I) 

 Frontal section of a 38-hour larva at the level of the notochord showing section through 

 the neural tube at the anterior and posterior ends, i.e., in region where larva bends 

 ventralwards. Thirteen pairs of somites are present with muscle fibrillae along the mesial 

 borders of the somites. 



