THE BNTBEOPNEtrSTA. 69 



Fig. 36. — Througli tlie anterior end of the nervous system (in the region 

 of the mouth), showing the manner in which the lumen arises in the 

 nerve-cord. 



Fig. 37. — Through the anterior part of the collar, showing the nearly com- 

 plete separation of the notochord with the hypoblast. 



Fig. 38. — Section taken behind the previous one. The nervous system is 

 here separated from the skin. 



Fig. 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. 



Fig. 40 is taken rather in front of the branchial sacs; it shows the two 

 anterior horns of the posterior body cavities, which form the perihsemal 

 cavities. 



Fig. 41. — Section through the gill-sacs in front of the clefts. The nervous 

 system here is fused to the skin dorsally. 



Fig. 42. — Through the extreme posterior end of tne second body cavities, 

 and the anterior end of the ventral blood-vessel and nervous cord. 



Fig. 43. — (Section not quite transverse.) Through the left gill-slit. 



Fig. 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. 



Fig. 45. — A longitudinal vertical section of the whole animal. (Two gill- 

 slits.) (The section is not truly vertical, as it cuts the gill-slit.) It exhibits 

 the relation of the notochord and other parts. (Obj. CO, oc. 2.) 



Fig. 46. — By an oversight no figure bearing this number appears in the 

 plates. The figure referred to as such in the text at pages 86, 98, and 103, 

 is Fig. 45. 



Fig. 47. — A transverse section of the extreme tip of the notochord, &c., 

 in the larva from which Figs. 31 — 45 were taken. It shows the histology of 

 the notochord and the arrangement upon it of the mesoblastic tissues, which 

 are pushed in by it. (Obj. D, oe. 3.) 



Fig. 48. — Transverse section across the proboscis pore of a larva with three 

 gill-slits. The two skeletal rods are here fused and have attained a considerable 

 size. (Obj. F, oc. 2.) 



Figs. 49 — 53 represent transverse sections taken from B. Kowalevskii, 

 at the stage of four pairs of gill-slits. (When the remaining structures are 

 dealt with a fuller explanation will be given of these and of the subsequent 

 figures ; on the present occasion they are only introduced to explain the account 

 of the notochord given in the text.) Figs. 49—53 are numbered from before 

 backwards. 



