422 MORGAN. [Vol. V. 



more distinct, and the longitudinal ciliated band disappears. 

 The lower limb of the latter marks the anterior end of the 

 collar. Serial sections show clearly the changes which have 

 taken place in the dorsal region during this period. 



The first change is shown by the transverse section shown in 

 Fig. 48. This is from a larva a little older than that shown by 

 Fig. 6, PL XXVII., but not so old as that shown by Fig. 46, 

 PI. XXVII. In the mid-dorsal line the ectoderm is seen to have 

 developed more of the nerve-fibre layer than elsewhere, and a 

 plate of this ectoderm is sinking a little below the general level 

 of the ectoderm. Figs. 49 and 50 are from older larvas at about 

 the stage shown in Fig. 46. Fig. 49 passes through the anterior 

 part of the neural plate, which is very broad, and at its two ends 

 the collar is seen rolling over the plate. Fig. 50 is about the 

 middle of same plate, the intermediate sections not being figured, 

 and here the plate is seen to be not nearly so wide as in the pre- 

 ceding figure, but to have sunken farther beneath the surface. 

 In both 49 and 50 the plate is broader than the average. The 

 series of figures 51-55 inclusive show a later stage in the devel- 

 opment of the chord when the collar has met and fused above 

 the middle line. Fig. 51 is the most anterior section, and in 

 this region there is always a small lumen in this part of the 

 chord. The fusion of the collar above the chord is complete. 

 Fig. 52 is from the middle of the chord, and here the lumen is 

 entirely obliterated, leaving a solid chord fused above with the 

 ectoderm. Fig. 53 is a section just back of the collar, where 

 the nerve chord is seen to be in process of formation by the 

 same changes that took place in the collar region. Figs. 54 and 

 55 give the more posterior sections through the chord, which is 

 here seen to be merely a thickened plate of ectoderm, not yet 

 beneath the surface. 



The account of the formation of the nerve chord given by 

 Bateson for B. Kowalevski differs in some points from the 

 above description. According to his account the middle part of 

 the chord simply delaminates from the ectoderm, but at the two 

 ends it arises by invagination. In the Tornaria development 

 we have seen the invagination takes place through the whole 

 length of the chord, in exactly the same way that the nerve chord 

 of Amphioxus is formed. 



A longitudinal section of the whole larva just after the meta- 



