Localized stages In development. ^1 



the life of the individual, or are cajjable of being brought into existence by certain condi- 

 tions throughout the life of the individual. Localized stages are essentially features of the 

 adult, although they may be evinced earlier or later than what wovdd technically be con- 

 sidered as the ephebic period. 



In Strassburger's Botany ('98) occurs a statement which is to the point in the 

 present studies. It is as follows : " Whatever is true of the development of a plant 

 from the embryo, is also, as a rule, applicaljle to its further growth from the growing point, 

 and, consequently, a knowledge of the mode of development at the growing point is of 

 great importance in detecting homologies." The same principle is applicable to many 

 animals. 



As stages found in the young repeat the characters of ancestral forms, so stages found 

 in localized parts repeat the characters of ancestral forms, and their adult equivalents are 

 to be sought in more primitive, sometimes more specialized, living or fossil allies. From 

 localized stages we may gather suggestive clews to the phylogeny of a group. 



Localized stages have an important bearing on the study of variation. In the arrested 

 or specialized development of a localized part, characters may be seen which are comparable 

 to more or less exceptional individuals, where the whole organism has an arrested or other- 

 wise modified character. The localized and the individual variation may be reversionary 

 or progressive in the line of variation of the group, and comparable to fossil or li\dng, 

 ancestral or specialized membei's of the group. From this point of view, localized stages 

 deserve careful stud}^ Besides atavic and progressive variation, aberrant variation as 

 well as parallelism may be represented in localized or individual variation. To which 

 class of variations a given character belongs, cannot usually be safely assumed without 

 a study of the young and related forms both living and fossil. 



In studying localized stages of developTnent, it is found tliat there are certain general 

 methods of their expression Avhich should be con.sidered. In the ontogeny of a localized 

 part, the stages may be transitory, and only seen if the part is observed at a certain period 

 of its growth. Such are the earlier stages in the growth of individual leaves, the simple 

 leaves at the base of suckers of many plants, as oaks, ashes, Ailanthus, the simplest con- 

 dition of budding corals and of the plates of crinoid stems and Echini. On the other hand, 

 localized stages in developn>ent may be permanently retained in certain areas of the part. 

 Cases of such local permanent retention of primitive features are the distal tips of leaves 

 which resemble the whole leaf of the young, as seen in the Tulip-tree and in the compound 

 leaves of Ailanthus, ashes, Phoenix, Areca, Pteris ; or the proximal portion of the leaf 

 may repeat the character of the simpler condition, as in Gymnocladus. The septal sutures 

 of Ammonites retain permanently on their inner umbilical border a simpler condition. 



The portion of the organism which is represented in the localized stage may be more 



