120 WILLIAM BATESON. 
Fic. 29.—The anterior ends of the third pair of body cavities are here cut 
as a solid mass of mesoblast on each side. 
Fic. 30.—A longitudinal vertical, nearly median, section of a larva, in the 
same stage as that shown in Figs. 21—29. The differentiation of the walls of 
the digestive tract may be here seen. 
Figs. 31—44 are from transverse sections of a larva which has just 
acquired the second pair of gill-slits. They are numbered from before 
backwards. 
Fic. 31.—Transverse section of the proboscis cavity. The loose tissue in 
the sac of the gland is shown. The membrane which is deposited round it is 
visible (ep. Fig. 47). (Obj. D, oc. 2.) 
Fic. 32.—Small portion of skin and mesoblast on a larger scale, to show 
structure of the nervous layer. (Obj. F, oc. 2.) 
Fie. 33.—Region behind that shown in Fig. 31. (Obj. D, oc. 2.) 
Fic. 34.—Through the proboscis stalk. The anterior horns of the middle 
body cavities are here cut. 
Fic. 35.—Through the proboscis stalk and the anterior phlange of the 
collar which forms the lower lip. 
Fie. 36.—Through the anterior end of the nervous system (in the region 
of the mouth), showing the manner in which the lumen arises in the 
nerve-cord. 
Fic. 37.—Through the anterior part of the collar, showing the nearly com- 
plete separation of the notochord with the hypoblast. 
Fic, 38.—Section taken behind the previous one. The nervous system is 
here separated from the skin. 
Fic. 39. Through the junction of the lumen of the notochord with that of 
the gut. The skeletal rods, which here bend downwards, backwards, and 
then slightly forwards, are therefore cut twice on each side. 
Fic. 40 is taken rather in front of the branchial sacs; it shows the two 
anterior horns of the posterior body cavities, which form the perihemal 
cavities. 
Fie. 41,—Section through the gill-sacs in front of the clefts. The nervous 
system here is fused to the skin dorsally. 
Fic. 42.—Through the extreme posterior end of the second body cavities, 
and the anterior end of the ventral blood-vessel and nervous cord. 
Fic. 43.—(Section not quite transverse.) Through the left gill-slit. 
Fie, 44.—Section taken through the extreme posterior end of the branchial 
region of the gut, showing how this overlaps the digestive region. ‘The dila- 
tation in the sides of the branchial region here shown are parts of the second 
pair of gill-slits. 
Fie. 45.—A longitudinal vertical section of the whole animal. (Two gill- 
