374 THE BLASTULA IN RELATION TO INNATE CONDITIONS 



powers of differentiation. To state the matter differently, the various, pre- 

 sumptive, organ-forming areas of the blastula have acquired different abilities 

 not only in their power to produce specific organs of the future body of the 

 embryo, but also in that some presumptive areas possess this propensity in 

 a greater degree than do other areas. However, at this point, certain terms in 

 common usage relating to the problem of differentiation are defined in order 

 that a better understanding may be obtained concerning the ability to dif- 

 ferentiate on the part of the presumptive, organ-forming areas of the late 

 blastula. 



B. Problem of Differentiation 

 1. Definition of Differentiation; Kinds of Differentiation 



The word differentiation is applied to that phase of development when a 

 cell, a group of cells, cell product experiences a change which results in a 

 persistent alteration of its activities. Under ordinary conditions an alteration 

 in structure or function is the only visible evidence that such a change has 

 occurred. 



To illustrate these matters, let us recall the conditions involved in the 

 maturation of the egg. A subtle change occurs within the primitive oogonium 

 which causes it to enlarge and to grow. This growth results in an increase in 

 size and change in structure of both the cytoplasm and the nucleus. A little 

 later, as the egg approaches that condition which is called maturity, observable 

 morphological changes of the nucleus occur which accompany or initiate an 

 invisible change in behavior. These latter changes make the egg fertilizable. 

 Here we have illustrated, first of all, a subtle, invisible, biochemical change 

 in the oogonium which arouses the formation of visible morphological changes 

 in the oocyte and, secondly, a morphological change (i.e., nuclear maturation) 

 which accompanies an invisible physiological transformation. 



Another illustration will prove profitable. Let us recall the development 

 of the mammary-gland tissue (fig. 58). Through the action of the lactogenic 

 (luteotrophic) hormone, LTH, the cells of the various acini of the fully 

 developed gland begin to secrete milk. The acini, it will be recalled, were 

 caused to differentiate as a result of the presence of progesterone. Similarly, 

 the various parts of the complicated duct system were stimulated to differen- 

 tiate from a very rudimentary condition by the presence of estrogenic hormone. 

 Earlier in development, however, the particular area of the body from which 

 the duct rudiments ultimately arose was conditioned by a change which dic- 

 tated the origin of the duct rudiments from the cells of this area and restricted 

 their origin from other areas. 



In the foregoing history of the mammary gland, various types of differen- 

 tiation are exemplified. The final elaboration of milk from the acinous cells 

 is effected by a change in the activity of the cells under the influence of LTH. 

 The type of change which brings about the functional activities of a structure 

 is called pfiysiological differentiation. The morphological changes in the cells 



