MAY 1 1899 



4. Localized Stages in Development in Plants and Animals. 

 By Robert Tracy Jackson. 



(Read November 2, 1898.) 



In this paper growth is considered in its Hmitation as mere increase in hulk, with- 

 out addition of differential characters. Development, on the other hand, is used in the 

 sense of implying ditferentiation, either progressive or regressive. 



In the ontogeny of animals, stages in development have long been recognized, and 

 their elucidation has been attained in numerous researches. Von Baer was one of the 

 first to recognize the fact, that early stages of the embryo resemble the adult condition of 

 simpler forms in the animal kingdom. Louis Agassiz accepted this view, and greatly 

 enlarged it. Professor Aga^^si^^ (03) gave full expression to the view, that stages found 

 in the ontogeny of the individual represent, in an epitomized form, the adults of simpler 

 types of the groujD to which the animal belongs. He first recognized the important 

 principle that the succession of stages in development of the individual represents, in an 

 abbreviated form, a series of adult more primitive types, and that this series of primitive 

 types appeared in geological succession in the same progressive order in which tlie repre- 

 sentative stages appear in the ontogeny of the indixddual. 



The important law of stages in ontogenesis was formulated by Prof. Alpheus Hyatt 

 as follows: In the yoimg, stages are found, the eqidvalents of ivhich are to he sought in the 

 adults of ancestral types. 



While the earliest stages in development have been extensively studied l>y emln-y- 

 ologists, later or post-embryonic stages, on which Professor Agassiz laid especial stress, 

 have been generally overlooked. It is for the most part in recent years that these later 

 stages have received the attention tliat is their due. The study of later stages has been 

 the principal method of research by Hyatt, Beecher, Clarke, Schuchert, Smith, the author, 

 and others. From this method of study important results have been attained in our 

 knowledge of the filiation of several groups of invertebrates. 



From his researches Professor Hyatt discovered the fact, that in the old age of the 

 individual, stages may also be found. These senile stages recall in a broad way the some- 

 what similar stages seen in the young. Successive senile stages, however, when repeating 

 larval or adolescent stages, do so in the hiverse order of succession from that of their 



