PATTERNS OF DEVELOPMENTAL PROGRESS 717 



many plant cells. If, for any reason, they resume division, they lose their 

 visible differentiation, though not necessarily their specificity; and the 

 earlier type of growth by synthesis of protoplasm may again appear. In 

 regeneration of striated muscle, for example, cytoplasm without visible 

 differentiation accumulates about the muscle nuclei, and spherical divid- 

 ing cells with protoplasmic growth result; later they cease dividing and 

 differentiate into muscle. Many other similar cases appear in reconstitu- 

 tions. 



In all except spherical organisms growth differs in rate and amount 

 in different directions and is the chief factor in determining the specific 

 forms of organisms and their parts. Different kinds of growth may occur 

 in different parts: some may grow by synthesis of protoplasm, others by 

 swelling, deposition of skeletal material, etc. Changes in rate and char- 

 acter of growth may also occur in a region or organ in the course of de- 

 velopment. In general, growth rate decreases with progress of develop- 

 ment, at least after early embryonic stages; but the decrease may be modi- 

 fied by growth cycles, by appearance of new growth centers, by meta- 

 morphosis, and by other factors. Differential growth of certain parts, the 

 hydroid tentacle, the sea-urchin archenteron, and the amphibian optic 

 primordium, becomes evident at certain developmental stages; that 

 of others, hydroid stolons, arms of the sea-urchin pluteus, and the am- 

 phibian limb, at other stages. In regenerative reconstitution growth rate 

 of regenerating parts is usually much above that of others. In starv- 

 ing planarians growth of the regenerating head and posterior end acceler- 

 ates reduction of the whole. Growth patterns of reconstitution by re- 

 organization without regeneration, as in Tuhularia and in Corymorpha, 

 do not show these extreme differences in growth but approach more nearly 

 embryonic patterns. Growth of protoplasm is very generally characteris- 

 tic of earlier embryonic stages and earlier stages of particular organs; as 

 development progresses, other kinds of growth appear. The plant axis 

 provides an interesting example of both spatial and chronological differ- 

 ences in growth. Growth by increase of protoplasm is characteristic of 

 earlier embryonic and bud stages and persists in the vegetative tip 

 throughout its growing life ; but as cells become farther removed from the 

 tip by its continued growth, protoplasmic growth decreases or ceases, 

 vacuolization begins, and most of the increase in size of the plant results 

 from enlargement of the vacuole, from formation of substances that swell 

 by uptake of water, from formation of cellulose, and from deposition in 

 storage organs of starch. But whatever the changes in rate and kind of 



