60 MORPHOLOGY OF 



Pies. 49 and 50 illustrate the histology of the anterior end of the notochord. 

 (Obj. i\ 00. 2.) These sections are in front of the heart. 



Fis. 51 shows a section of the whole proboscis, to illustrate the relations of 

 the parts. The body cavity may be seen to be nearly filled up with muscle- 

 fibres and connective tissue, with the exception of a small central space, in 

 which lies the notochord bearing the central mesoblastic structures, viz. the 

 heart and proboscis gland with its sac. (Obj. C, oc. 2.) 



EiG. 52 exhibits the central structures of Fig. 51. (Obj. P, oc. 2.) 



Fig. 53. — Here the lumen of the notochord is reached. At this point the 

 sac of the proboscis gland is attached dorsally to the skin of the proboscis. 

 (Obj. F, oc. 2.) 



Fig. 54. — Transverse section of proboscis stalk of an older animal, taken 

 beliind the epiblast sac of the proboscis pore, the back of which appears in 

 section. The skeletal rod has still in this region two central "cores." (Obj. 

 D, oc. 2.) 



Fig. 65. — A transverse section of the notochord of an adult B. Kowa- 

 levskii, taken in front of its lumen (cp. Fig. 57). (Obj. CO, oc. 2.) 



Fig. 56. — Transverse section of the same as foregoing, through the dilated 

 lumen in front of the skeletal rod. 



Fig. 57. — Longitudinal vertical median section of the back of the proboscis 

 and the front of the collar of B. Kowalevskii, to show the relation of the 

 notochord. The other parts are diagrammatic. (Obj. A, oc. 2.) 



Fig. 58. — Transverse section of the junction of the proboscis stalk with 

 the collar. (Semi-diagrammatic.) Exhibits relations of the notochord and its 

 sheath to the skeletal rods, nervous system, &o. (Obj. A, oc. 2.) 



Fig. 59. — A diagram of a transverse section taken through the anterior 

 region of the collar, to show the relation of the parts figured in Fig. 60. 

 (Obj. A, oc. 2.) 



Fig. 60.— The dorsal structures of the anterior part of the collar shown in 

 Fig. 59, viz. the nervous system, notochord, perihsemal cavities, &o. (The gut 

 is torn away from the skeletal rod, presumably by shrinking.) The lateral rods 

 are here connected to the notoohordal sheath. (Obj. D, oc. 2.) 



Figs. 61 — 63 show transverse sections of parts of B. minutUs. 



Fig. 61. — The notochord in transverse section in front of its lumen. (Obj. 

 OC, 00. 2.) 



Fig. 62. — The same in the region of the proboscis stalk. 

 Fig. 63.— Through the notochord, &c., in the front of the collar, showing 

 the thickened dorsal half of the organ. (Obj. CC, oc. 2.) 



