342 PATTERNS AND PROBLEMS OF DEVELOPMENT 



and gradient in the outgrowing tissue. That the nervous influence is not 

 specifically formative is shown by experimental alterations of the course 

 of brachial or sciatic nerves. Nerves thus diverted may determine Hmb 

 development at other than the normal position within the limb field ; but 

 the sciatic nerve, led into the tail region, determines a tail.^ Transplanted 

 primordia of chick appendages develop irrespective of innervation (Ham- 

 burger, 1939), but this development does not involve origin of a new 

 dominance. 



SERIAL HETEROMORPHOSIS OR HOMEOSIS 



Among arthropods regeneration of an appendage characteristic of a 

 particular segment usually results in an organ more or less similar to that 

 removed, though perhaps not completely normal; but sometimes an ap- 

 pendage resembling more or less closely that of another segment, usually 

 the next posterior, develops." In the case of development of antenna-like 

 structures in place of eyes, as observed in various Crustacea, Herbst, 

 Janda, and Kfizenecky find that this type of regeneration occurs after 

 removal of the optic ganglia, even though only the distal part of the eye 

 stalk is removed. Removal of antennae near the base apparently favors 

 regeneration of leghke structures; but, according to Suster (1933), normal 

 regeneration of antennae is possible in the orthopteran S phodromantis 

 after removal of the ganglia. In all these cases the regenerated structure, 

 when not normal, resembles the appendage of a segment posterior to 

 that on which it regenerates. In the light of the data on reconstitution 

 in hydroids and planarians and on differential modification of develop- 

 ment by external agents, it may be suggested that here, also, degree or 

 intensity of activation of cells following section is an essential factor in 

 determining the segmental character of the regeneration. If the ganglion 

 most directly related to the part is present, it is probably a factor in 

 bringing about sufficient activation to determine normal segmental regen- 

 eration; in absence of the ganglion there is less activation and in many 

 cases no regeneration, or an appendage may regenerate, but its cells fail 

 to attain the physiological level necessary for development of an ap- 

 pendage normal to the segment concerned, and a structure resembling 



'Locatelli, 1925; Guyenot et Schotte, 1926. Guyenot, 1928, reports similar results in a 

 reptile, Laceria. 



"Antenna-like organs in place of eyes: Herbst, 1896, b, c, 1899, 19016; Janda, 1913; 

 Kfizenecky, 1913. Leglike organs in place of antennae: Friza and Przibram, 1910; Przibram, 

 1910, 1917, 1919a, b; Brecher, 1924; Borchardt, 1927; Suster, 1933. Crayfish legs, Zalpeter, 

 1927. Homeosis in general, Przibram, 1910, 1920. 



