64 AMPHIOXUS. 



then grooved ventrally along the median plane (Fig. 28). The 

 ventral groove deepens, and at a stage with five pairs of meso- 

 blastic somites the plate is completely folded on itself, so that 

 its two halves are in contact with each other. The cells of the 

 two halves now begin to grow across the median plane, inter- 

 digitating with one another (Fig. 29, CH), and forming a solid 

 ridge of cells along the mid-dorsal surface of the archenteron. 

 At a slightly later stage, with eight or nine pairs of mesoblastic 

 somites, this ridge begins to separate from the gut wall as a 

 cylindrical rod of cells, the notochord (Fig. 32, CH). 



Behind the first somite, i.e. along the greater part of its 



MS r- - mr 



CH 



FIG. 28. FIG. 29. 



FIGS. 28 and 29. Transverse sections through Amphioxus embryos shortly 

 after the time of hatching ; showing stages in the formation of the noto- 

 chord and mesoblastic somites, x 435. (After Hatschek.) 



Fig. 28. Embryo with five pairs of somites : transverse section through the middle 

 of the first pair. Pig. 29. Embryo with six pairs of somites : transverse section through 

 the hinder end of the first pair. CE, enteroccelic pouch or mesoblastic somite. 

 CH, notochord. Gr, archenteron. MS, mesoblastic somite. NG-, neural canal, 

 T, mesenteron. 



length, the notochord develops from before backwards. Op- 

 posite the first somite the notochord forms more slowly, and is 

 always a little behind the stage reached in the second somite. 

 In front of the first somite the notochord is developed from be- 

 hind forwards, but otherwise in the same manner as in the hinder 

 part, though much more slowly ; towards the close of the em- 

 bryonic period, at the time when the pointed anterior end of the 

 animal is forming, it grows much more rapidly (Fig. 33). This 

 late development of the anterior end of the notochord will be 

 referred to again further on. 



