330 INTRODUCTION TO CYTOLOGY 



pattern of a jellyfish, starfish, worm, mollusk, insect or vertebrate are 

 foreshadowed by the characteristic polarity, symmetry and pattern of the 

 cytoplasm of the egg either before or immediately after fertilization" 

 (Conklin, p. 172-5). That the arrangement of the embryonic parts is 

 not solely dependent upon such visible egg substances is shown by the 

 observations of Morgan (1909c, 1910e) and Boveri (19106) on centrifuged 

 eggs of Arbacia, Ascaris, the frog, and other forms. Here it is found 

 that the displacement of the various substances does not necessarily 

 cause a dislocation of the body parts of the embryo: hence the setting 

 apart of the embryonic regions must be dependent upon a polarity in the 

 egg which at least in many cases is not disturbed by the experimental 

 alteration in position of the visible egg substances. But in either case 

 differentiation appears to be related to a cytoplasmic organization. 

 From this it would appear that the characters which such an organism 

 inherits from the preceding generation do not belong to one category and 

 are not transmitted in the same way. There are first those general 

 characteristics of organization which are the direct outgrowth of a corre- 

 sponding organization in the egg cytoplasm. Secondly, there are the 

 Mendelian characters which appear later in the ontogeny and which there 

 is every reason to believe are represented in some way in the chromosomes 

 of the gamete nuclei (Chapter XV). Boveri thus distinguished two 

 periods after fertilization: an early one in which the course of develop- 

 ment is dependent on the organization of the egg cytoplasm, only general 

 metabolic functions of the chromosomes being active; and a later one in 

 which the specific hereditary powers of the chromosomes are brought to 

 expression, the right chromosomal combination then proving to be 

 necessary for normal development. 



The question naturally arises as to how much the cytoplasmic organ- 

 ization may be due in turn to the activity of the nucleus during the 

 differentiation of the egg — as to whether the general characters which are 

 the direct outgrowth of this organization may or may not be ultimately 

 dependent, as are the clearly Mendelian characters, on nuclear factors. 

 Conklin (1915) comments upon this point as follows: "In this differentia- 

 tion and localization of the egg cytoplasm it is probable that certain 

 influences have come from the nucleus of the egg, and perhaps from the 

 egg chromosomes. There is no doubt that most of the differentiations of 

 the egg cytoplasm have arisen during the ovarian history of the egg, and 

 as a result of the interaction of nucleus and cytoplasm; but the fact 

 remains that at the time of fertilization the hereditary potencies of the 

 two germ cells are not equal, all the early stages of development, includ- 

 ing the polarity, symmetry, type of cleavage, and the pattern, or relative 

 positions and proportions of future organs, being foreshadowed in the 

 cytoplasm of the egg cell, while only the differentiations of later develop- 

 ment are influenced by the sperm. In short the egg cytoplasm fixes the 



