12 PATTERNS AND PROBLEMS OF DEVELOPMENT 



role in determining order, scale of organization, and growth forms of mul- 

 tiaxiate organisms; of its relation to induction; of the significance of physi- 

 ological isolation in development; and the factors involved in different 

 organisms and under different conditions — these are some of the questions 

 involved in the problem of physiological integration. 



THE MATERIAL OF DEVELOPMENTAL PHYSIOLOGY 



Comparative investigation of embryonic development in difTerent or- 

 ganisms and groups has given us a broader conception of this form of de- 

 velopment than could have been obtained from a single species or group. 

 Comparative study and experimental analysis of different forms of devel- 

 opment must provide a basis for a still broader conception and assist us 

 in distinguishing generally essential factors from those concerned with a 

 particular form of development. In all forms of development physiological 

 order, pattern, and integration appear, and evidence is accumulating to 

 show that the essential differences in the different forms are not as great 

 as has often been believed. Some of them present possibilities of experi- 

 mental analysis which are present to a lesser degree in embryonic develop- 

 ment. Sometimes it is even desirable to interpret embryonic development 

 in the light of what we learn from other forms of development. 



Embryonic development, development from spores, gemmules, stato- 

 blasts, and other special reproductive bodies formed under natural condi- 

 tions occur in cells or cell masses which have, in each case, a particular 

 past history which is always essentially the same in each species. Some 

 of these reproductive bodies appear to be highly differentiated cells or cell 

 groups, the egg and spermatozoon (the most highly differentiated of all) 

 having undergone greater change from the undifferentiated or embryonic 

 cell than most other cells of the individual. There can be no doubt that 

 the past history of the egg plays some part in determining the course of its 

 development, at least in the earlier stages. In many species there is a con- 

 siderable degree of regional differentiation in the egg cytoplasm preceding 

 or following fertilization; when present, this may become an important 

 factor in determining the manner in which development takes place, even 

 though the resulting individual may be similar to an individual which de- 

 velops from a physiologically or physically isolated part of the adult ani- 

 mal or from a bud. 



The reproductive cells known as "spores" arise in some unicellular or- 

 ganisms by fragmentation of the individual, but in multicellular organisms 

 they usually originate only under certain physiological conditions and 



