68 MORPHOLOGY OF 



tlie first cavity are all cut in this section. (The irregular folds of skin 

 are due to the contractions of the body ; it will be understood by comparing 

 the figures of the whole animal that only the central part within the folds is 

 the real stalk of the proboscis.) 



Fig. 19. — The section crosses the end of the archenteron, lying in front of 

 the mouth. (The whole of this part of the archenteron becomes eventually 

 pushed forward to form the notochord.) 



Fig. 20. — The mouth is here traversed, as also the anterior end of the 

 nervous system. (A space, due probably to shrinking, is visible in the dorsal 

 mesentery.) 



Figs. 21 — 29 are transverse sections of a larva, slightly older than the fore- 

 going. Fig. 25 was drawn under Zeiss's Immersion 2 and oc. 2 ; the others were 

 drawn under Obj. D and oc. 2. They are numbered from before backwards. 



Fig. 21. — Section taken just behind the mouth. The lumen of the noto- 

 chord is here shut off from the archenteron. 



Fig. 22.— The notochord still open to the archenteron. 



Fig. 23. — The nervous system is attached to the skin. 



Fig. 24. — The nervous system is already nearly separated from the skin. 



Fig. 25. — Part of the foregoing enlarged, to show the peculiar pyriform 

 cells of the splanchnopleure (*•). 



Fig. 26. — Nervous system still in the skin. 



Fig. 27. — The nerve-cord is separated from the skin. 



Fig. 28. — Two parts of the middle body cavities may be here seen separat- 

 ing from the rest, probably forming part of the perihsemal cavities. 



Fig. 29. — The anterior ends of the third pair of body cavities are here cut 

 as a solid mass of mesoblast on each side. 



Fig. 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 — a are from transverse sections of a larva which has just 

 acquired the second pair of gill-slits. They are numbered from before 

 backwards. 



Fig. 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 (cp. Fig. 47). (Obj. D, oc. 2.) 



Fig. 32. — Small portion of skin and mesoblast on a larger scale, to show 

 structure of the nervous layer. (Obj. F, oc. 2.) 



Fig. 33. — Region behind that shown in Fig. 31. (Obj. D, oc. 2.) 



Fig. 34. — Through the proboscis stalk. The anterior horns of the middle 

 body cavities are here cut. 



Fig. 35. — Through the proboscis stalk and the anterior phlange of the 

 collar which forms the lower lip. 



