262 GENERAL EMBRYOLOGY 



conditions are involved. The reactions of the ovum in cleaving, 

 of the blastula in gastrulating, and the like, are of a general 

 nature, i.e., the external conditions involved may be consider- 

 ably varied, within limits, and yet produce the same response 

 on the part of the organism. Emphasis thus is placed upon the 

 internal factors in the reactions of development, for on account 

 of the definite character of habits of life and of spawning, the 

 normal external conditions of development are sufficiently 

 uniform to produce a series of reactions, a development, which 

 is also uniform, i.e., normal. Of course it is easily possible to 

 alter, artificially, the external as well as the internal conditions 

 of development and the results of such alteration often lead, 

 as we shall see, to important ideas regarding normal or char- 

 acteristic embryonic behavior. 



Development is, then, a series of reactions, one condition 

 leading to the next; and the primary factor in determining the 

 quality of each reaction is the internal condition or structure, 

 both morphological and physiological, of the organism, whether 

 it be ovum, zygote, blastula, larva, or adolescent individual. 



Throughout the efforts to solve the problem of individual 

 development, the attempt has always been to explain existing 

 differentiations as being dependent upon some preexisting 

 differentiation, related but of a different kind. Thus at one 

 time the earliest differentiations that became visible in the 

 developing organism were the germ layers, and these were 

 consequently regarded as the fundamental differentiations 

 of the embryo, determining its subsequent history. Next, 

 differentiations among the cleavage cells were noted and 

 emphasized as primary. Then as technique improved, and 

 the subject of cytology developed, attention became focussed 

 upon the nucleus and its organs not only as the centers of cell 

 life, but as the structures primarily concerned in the differen- 

 tiations of the developing egg cell. And lately, chiefly as a 

 result of experimental analysis of the processes of development, 

 rather than as the consequence of observation alone, structural 

 differentiations of the cytoplasmic portion of the ovum have 

 occupied the center of interest as determining factors in 



