J 38 ROBERT TRACY JACKSON ON 



are formed first in the development of the embryo, they have consequently passed 

 through more molts than the posterior segments. To this greater number of molts he 

 ascribes the fact that the anterior appendages are more modified. The posterior append- 

 ages, have passed through fewer molts, and have retained the more primitive type of 

 limb. In this regional distribution of primitive and specialized types of limbs it seems 

 that we have another case of localized stages in development. 



Vertebral columx. In the development of the vertebral column, as I am in- 

 formed by Dr. Charles Sedgwick Minot, there is evidence of localized stages in develop- 

 ment similar to what I have shown in the plates of Echini. In the development of the 

 vertebral column the neck vertebrae are formed first, and from this point additional seg- 

 ments appear both anteriorly and posteriorlv- lu the caudal vertebrae they originate as 

 did the neck vertebrae and during development pass through ontogenetical stages similar 

 to those which have been previously passed through by the more advanced segments 

 of the column. The suljject, however, is one which calls for more exact investigation 

 than has yet been made. 



General Conclusions. 



As pointed out by Lubbock ('92), in seedling plants the cotyledons are usually more 

 or less strikingly different from the nepionic leaves. They are usuall}? simple in outline, 

 or if lobed, such lobes are usually slight (Eucharidium, Figs. 1-.3, page 103) ; serrations 

 are rare, and cotyledons are usually free from hairs or color markings, although such 

 features may occur in immediately succeeding nepionic growth. This sharp transition 

 from the last embryonic or so-called phylembryonic stage to the nepionic is in accord with 

 what has been observed in many animals at a similar stage. This sharp transition has 

 been shown by H_yatt and others in cephalopods, by Beecher in brachiopods and tri- 

 lobites, and l)y myself in pelecypods. In some cases cotyledons during growth may 

 take on nepionic characters, as in Eucharidium, Figs. 1-3, p. 103, and Clarkia. Coty- 

 ledons may by acceleration of development acquire some nepionic characters, as noted 

 in serrate spines in cotyledons of the White Pine (PI. 21, fig. 79a). 



Nepionic leaves immediately succeeding the cotyledons are usually simple, even 

 if the adults are compound; or at least simpler than adult leaves, as noted by Lubbock. 

 Amongst known fossil representatives of living plants, these more or less ancient types 

 are usually simpler (ban modern allies. Nepionic leaves of seedlings, therefore, as 

 representing ontogeny, may be taken in general as repeating in an epitomized form 

 the characters of the adults of ancient fossil allies of the type. Exceptions to this may 

 occur in plants as in animals, where early stages are modified by youthful or larval 

 adaptations. Other exceptions may occur in plants as in animals, which are modified 



