Ventral membrane (composite) 



Dorsal membrane 



Spinal cord 



Notochord 



Spinal cord 



Notochord - - :, 

 Dorsal membrane '" 



Ventral membrane (composite) ~ 



FIG. 30. Transverse section through tail 

 of embryo belonging to the type illustrated 

 in PI. II. fig. 12 (ventral union by posterior 

 part of body). There are two spinal cords, 

 notochords, and dorsal edge membranes. 

 Two ventral edge membranes are also 

 present, but these are composite, being 

 derived from the widely separated halves 

 of ventral membranes belonging to each 

 of the twins. This is an example of 

 almost pure ventral union. See pp. 22-23. 

 Compare with Fig. 31. (xi4.) 



Dorsal membrane 

 Muscle segments 

 Spinal cord 

 Notochord (composite) 



Aorta 



Caudal vein 

 Ventral membrane 

 Spinal cord 



FIG. 31. Transverse section through 

 tail of an embryo belonging to the 

 general type illustrated in PI. VI. figs. 

 27-29. The two notochords have fused 

 together, and the ventral edge membrane 

 is composite and single. Compare with 

 Fig. 30. See also under Fig. 27. (x 14.) 



l. Tran>versr section through com- 

 : tile tail of the triple monster 

 trout described on pp. 33-34 See also PI. 

 XX11. figs. oj-96and PI. XXIII. figs. 97-99. 

 The section is taken up chiefly with structures 

 belonging to the principal embryo, but it 

 shows also, near the middle line on the under 

 aspect, the two dorsal fins of the defective em- 

 bryos. A little further up on either side is a 

 composite ventral edge membrane (anal fin), 

 belonging in part to the principal embryo avid 

 10 one or other of the defect! ve embryos. 



Muscle segments of principal embryo 

 Spinal cord ,, ,, 



Notochord 

 ...... Aorta 



Caudal vein 



Muscular tissue of defective embryos 



$- Ventral membrane (composite) 



Dorsal membrane of defective embryo B 



A 



Dorsal membrane 

 Muscle segments 



Spinal cord 

 Notochord 



Ventral membrane (composite) 



Ventral membrane (composite) _ 



Dorsal membranes of \ 

 embryos A and B / 



Naming of parts same as in 

 last figure. The dorsal mem- 

 branes of A and B are now 

 united. 



l'n;. 33. Transverse section through tail of 

 same specimen as in last figure, but considerably 

 further back. All the fin membranes are still 

 present, but the two dorsals of the defective em- 

 bryo have almost united. The section, as well 

 as the succeeding one, may be compared with 

 Figs. 30 and 31. Other references under Fig. 32. 



FIG. 34. Transverse section as above. The 

 dorsal fins of the defective embryos hnve now 

 united. (xi8. ) 





J. F. G. 



PL. VII. 



vn 



