TUBULATION OF ORGAN-FORMING AREAS IN AMPHIOXUS 505 



The formation of a continuous, antero-posterior, coelomic cavity in Am- 

 phioxus may be described as follows. The mesodermal bands on either side 

 of the notochord of the post-gastrular embryo become converted into meso- 

 dermal grooves as each mesodermal band folds inwards or evaginates into 

 the residual blastocoelic space between the archenteron and the outside ecto- 

 derm (fig. 195). Beginning at the anterior end, these longitudinal grooves 

 of mesoderm soon become divided into distinct segments or somites by the 

 appearance of transverse divisions (fig. 249 A, B, H). The first and second 

 pairs of somites are formed at the anterior ends of the mesodermal grooves 

 at about the time that the embryo hatches and swims about by means of 

 ciliary action. 



Eventually each somite becomes entirely constricted from the notochord 

 and entoderm. In this segregated condition the somite forms a rounded struc- 

 ture retaining within itself a portion of the original archenteric cavity (fig. 

 250A). Hence, the cavity within the somite is called an enterocoel and repre- 

 sents the beginnings of the coelomic cavity of later development, at least in 

 the anterior 13 or 14 pairs of somites. {Note: It is to be observed in this 

 connection that the primitive somite in Amphioxus is not comparable to the 

 primitive somite of the vertebrate embryo. In the latter, the somite represents 

 merely a segment of the epimeric mesoderm, whereas in Amphioxus it is the 

 entire mesoderm in each half of a particular segment of the embryo.) 



After hatching, the mesodermal bands continue to form into grooves as the 

 embryo elongates, and, synchronously, successive pairs of somites are formed. 

 At about the time 8 to 10 pairs of somites are present (fig. 249B, H), the 

 enterocoels of the first two pairs of somites have become entirely separated 

 from the archenteron. The enterocoels of the following six pairs of somites 

 are small and are not as evident at first as those of the first two pairs. Ulti- 

 mately a definite enterocoel is found, however, in each somite. 



Posterior to the eighth or ninth pairs of somites, the forming mesodermal 

 grooves do not show the enterocoelic pouches as plainly as the more anterior 

 somites. Slit-like mesodermal grooves tend to be present, however, and, when 

 the somite is entirely free from the archenteron, this slit-like cavity expands 

 into the enterocoelic space of the somite. As the region of the fourteenth 

 pair of somites is approached, the slit-like mesodermal groove becomes more 

 and more indefinite. Posterior to the fourteenth or fifteenth pair of somites, 

 the somites originate from a solid mesodermal band on either side of the 

 notochord. An enterocoelic origin of the cavity within each somite, therefore, 

 is not possible caudal to this area, and the coelomic space arises by a 

 hollowing-out process similar to coelomic cavity formation in the vertebrate 

 group. 



At about the time when eight pairs of somites are established, a shift of 



