426 MORGAN. [Vol. V. 



mid-ventral part is seen the notochord (nc). It contains a nar- 

 row central cavity which is continuous with the cavity of the 

 digestive tract. The walls are formed by large vacuolated cells 

 with the protoplasm especially collected near the lumen. Beneath 

 the notochord is a single chitin-like rod which is formed by the 

 union of the two rods found in the folds of the digestive tract. 

 Above the notochord, in the middle line at h y lies the proboscis 

 blood-vessel (heart). This is bounded above by the proboscis 

 vesicle (v), and at the sides by the right and left horns of the 

 anterior enteroccel (e), thus preserving the same relations found 

 in younger stages. The cells lining the walls of the enteroccel 

 begin to show muscular differentiations in their outer ends. 

 The proboscis vesicle is lined by a single layer of cells. A sec- 

 tion farther forward through the proboscis is shown by Fig. 61. 

 The ectoderm is still comparatively thick, with quite a wide zone 

 of the nerve-fibre layer. The notochord (nc) is still present and 

 similar to that in the last section. The chitin rod does not 

 extend so far forward. The two horns of the enteroccel have 

 united into the single large anteroccel. The walls of the latter 

 are proliferating, so that the cells encroach upon the central 

 cavity. At v is seen the ending of the proboscis vesicle. The 

 walls of the heart (h) hang from this point into the central 

 cavity, so that the heart is seen to run across from the lower to 

 the upper wall of the enteroccel, as shown in the earlier stages. 

 Fig. 63 shows part of a section between Figs. 60 and 61, and 

 shows the point at which the cavity of the heart passes across. 



A section through the middle of the proboscis is shown in 

 Fig. 62. The ectoderm is not so thick nor does it contain so 

 broad a zone of the nerve-fibre layer. The centre is filled by 

 the anterior enteroccel, whose walls are beginning to form the 

 musculature, the proboscis and the cells by proliferating to fill 

 up the central cavity. 



We may next examine the sections posterior to the collar. 

 Fig. 64 is from a section passing through the first pair of gill 

 pouches. The ectoderm is thick and ciliated, but does not show 

 the gland cells found in the collar region. Beneath the ecto- 

 derm in the mid-dorsal line lies the solid nerve chord. The 

 digestive tract has on each side of the middle line two enor- 

 mous pouches which are richly ciliated where they join the gut. 

 They have not yet come into contact with the ectoderm, but 



