TUBULATION OF ORGAN-FORMING AREAS 473 



around the caudal end or base of the notochord. In the developing frog on the 

 other hand, the confluence between the neural and gut tubes is present only dur- 

 ing the initial stages of tail formation, and it thus represents a transient relation- 

 ship (fig. 223B, C). Consequently, as the tail bud in the frog embryo grows 

 caudally, the neurenteric connection is obliterated and the tail gut disappears. 

 On the other hand, in the European frog, Bombinator, the condition is inter- 

 mediate between frog and shark embryos (fig. 228). True neurenteric canals 

 within the developing tail are never formed in the reptile, chick, or mammal, 

 although a tail or post-anal gut, much abbreviated, develops in these forms. 

 (See paragraph below.) In teleost fishes, Kupffer's vesicle possibly represents 

 a small and transient attempt to form a neurenteric canal (fig. 210G). How- 

 ever, the tail gut here, with the exception of the terminally placed Kupffer's 



Fig. 227. Stages of normal development of Necturus maculosiis. (Slightly modified 

 from Eycleshymer and Wilson, aided by C. O. Whitman; Chap. 11 in Entwicklimgs- 

 geschichte d. Wirbeltiere, by F. Keibel, '10.) (A) Stage 15, 14 days, 19 hours after 

 fertilization. Blastopore is circular and reduced; neural groove is indicated in center 

 of figure. (B) Stage 18, 17 days, 2 hours old. Blastopore is an elongated, narrow 

 aperture between caudal ends of neural folds; neural folds prominent and neural groove 

 is deeper. (C) Stage 21, 18 days, 15 hours old, 3 or 4 pairs of somites. Neural folds 

 are widely separated in head region, narrower in trunk, and coalesced in tail area. (D) 

 Stage 22, 20 days, 10 hours, 6 pairs of somites, length about 6 mm. Observe head has 

 three longitudinal ridges, the middle one represents developing brain, while lateral ones 

 are common anlagen of optic vesicles and branchial arches. (E) Stage 23, 21 days, 2 

 hours, 10 to 12 pairs of somites, 7 mm. long. Head projects forward slightly above egg 

 contour; end of tail is prominent; large optic vesicles protrude laterally from head 

 area; branchial arch region is caudal to optic vesicle enlargement; anus is below tip of 

 tail. (F) Stage 24, 22 days, 17 hours, 16 to 18 pairs of somites, 8 mm. long. Anterior 

 half of head is free from egg contour; optic vesicles and mandibular visceral arch are 

 well defined. (G) Stage 25, 23 days, 10 hours, 20 to 22 pairs of somites, 9 mm. long. 

 Head is free from egg surface; t^il outgrowth is becoming free; mandibular, hyoid, first 

 branchial and common rudiment of second and third branchial arches are visible. Otic 

 vesicle lies above hyoid arch and cleft between hyoid and first branchial arches. (H) 

 Stage 26, 24 days. 22 hours, 23 to 24 pairs of somites, length^ 10 mm. Head and caudal 

 outgrowths are free from egg surface; heart rudiment is shown as darkened area below 

 branchial arches; cephalic flexure of brain is prominent. (I) Stage 27, 26 days, 26 to 

 27 myotomes, length — 11 mm. Outline of body is straighter; nasal pits and mouth are 

 well defined, mandibular arches are long; heart is prominent below branchial arches; 

 anterior limb buds are indicated; faint outlines of posterior limb buds are evident. (J) 

 Stage 28, 30 days, 8 hours, 30 to 31 myotomes, length — 13 mm. Trunk of embryo is 

 straight, head and tail are depressed; surface of yolk is covered by dense network of 

 capillaries; vitelline veins are prominent; pigment appears below epidermis; anterior limb 

 bud projects dorsally; nuchal or neck flexure is prominent above heart and limb-bud 

 area. (K) Stage 29, 36 days, 16 hours, 36 to 38 myotomes, length — 16 mm. Mandibular 

 arches are forming lower jaw; nuchal and tail flexures are straightening; eye and lens 

 are well defined; anlagen of gill filament are present on gill bars; pigment cells are 

 evident on head areas; vitelline veins are prominent; yolk-laden, ventro-lateral portion of 

 trunk is becoming elongated and contracted toward dorsal region of embryo. (L) Stage 

 30, 40 days, 20 hours, 44 to 46 myotomes, length — 18 mm. Fore and hind limb buds are 

 prominent; nasal openings are small. (M) Stage 31. larva 49 days, 21 mm. (N) Stage 

 32, larva 61 days, 25 mm. (O) Stage 33, larva 70 days, 28 mm. (P) Stage 34, larva 

 97 days, 34 mm. (Q) Stage 35, young adult form, 126 days, 39 mm. 



