EXTIRPATION OF ORGAN ANLAGEN 



251 



THE URODELE BALANCERS 



The balancers are slender rod-like appendages which project from the side of the head, 

 slightly posterior and ventral to the eyes, and which serve as supports to hold the head 

 off the bottom and maintain balance in many Urodeles (see Amblystoma series stage #40). 

 They are present in most species of Triturus, in Amblystoma punctatum (maculatum), 

 jeffer sonianum, microstomum and opacum but are absent in A. tigrinum. The club- 

 shaped ends of the fully formed balancers are sticky, and this condition may be taken as 

 a criterion of normal development. 



Select Amblystoma punctatum or opacum at stages #28 to #32 and extirpate the balancer 

 anlage on the right side, leaving the left side as the control. Remove both the ectoderm 

 and the underlying mesoderm from a circular area posterior and slightly ventral to the 

 eye, on the mandibular arch. The dorsal margin of the balancer area is on a direct line 

 from the dorsal border of the eye, and the area extends to a level below that of the eye. 

 The area also extends from the posterior margin of the eye to the first gill slit. 



THE EXTERNAL GILLS 



Select Anuran embryos at stage #17 or #18 or 

 Amblystoma embryos (any species) at stage #25 or 

 #26 and locate the gill swellings. If available, 

 study Anura stage #23 and Amblystoma stage #40 

 to see the position of the filamentous external gills 

 which normally develop from the anlagen. 



Since all three germ layers contribute to the for- 

 mation of the external gills, it will be necessary 

 to extirpate an adequate area of the ectoderm and 

 a considerable amount of the underlying meso- 

 derm of these early stages in order to make the 

 operation complete. This may be done with glass 

 needles, and deep excavation by means of a hair 

 loop. The pharyngael cavity should be exposed. 

 The gill anlage on the right side may be removed 

 and the left side left untouched for control com- 

 parison. 



Position of the balancer anlage' (stippled) 

 and posterior to it the gill swelling: 

 Amblystoma stage #28. 



THE EYE FIELD OF THE NEURULA 



Manchot (1929), Adelmann (1930) and Mangold (1931) have shown that the two eye fields 

 are located close together near the median line of the anterior medullary plate of stage 

 #14 or #15 of the Urodela or stage #14 of the Anura. The eye field extends beyond the 

 ultimately realized eye area, and extirpations and transplantations often result in pro- 

 ducing accessory eyes. 



Attempt to extirpate the eye forming area of the right eye alone. With glass needles cut 

 deeply along lines indicated on the accompanying diagram, exposing the archenteron be- 

 neath. Remove the ectoderm and the substrate of this area while the embryo is in 10% 

 Standard Solution and allow it to heal completely before gradually returning it to the full 

 strength Standard Solution. It will be necessary to operate on a number of embryos be- 

 cause survival is low. 



Variations of this extirpation should be in the direction of widening the area toward the 

 left side; moving the area toward the mid-line; removing the epidermis with and without 

 the substrate. (See section on Eye Field Operations. ) 



