276 



ANIMAL BIOLOGY 



chromosomes — contributed jointly by the two gametes. To this 

 extent, then, the nucleus and therefore the egg exhibits a ready- 

 formed structural basis which (as we have already suggested, and 

 will have occasion to elaborate later) certainly is definitely related 

 to characters which appear in the offspring. 



Turning to the egg cytoplasm, we are confronted with conditions 

 which are not so uniform but nevertheless highly suggestive. In 



Fig. 178A. — Egg of a Mollusc, Dentalium, showing cytoplasmic differentia- 

 tion. A, egg, shortly after being extruded and before maturation is completed, 

 showing three differentiated regions; B, section through an egg after fertiliza- 

 tion, showing cytoplasmic rearrangement involving the segregation of clear po- 

 lar lobe at p; C, normal sixteen-cell stage, with materials of polar lobe now in 

 cell X. Bemoval of the polar lobe results in an abnormal embryo. (After 

 Wilson.) 



the first place, before fertilization the egg possesses a definite 

 polarity, expressed, for example, by the position of the nucleus 

 and the distribution of food material (yolk), pigment granules, 

 and vacuoles. This polarity is traceable, in part at least, to the 

 polarity of the oogonia, and through them to the germinal epi- 

 thelium. In brief, the egg as a whole is organized; the invisible 

 organization of the fundamental matrix of the cytoplasm being 

 revealed, in part, by the disposition of various elements of the 

 cell. Now this cytoplasmic organization undergoes more or less 

 profound changes in establishing that of the new individual. In 

 some cases the reorganization occurs at fertilization, while in 

 others it is somewhat deferred. And herein, apparently, is to be 

 sought the explanation of the difference in behavior ■*— in potential- 

 ities - - of various types of eggs during cleavage stages. Two con- 

 trasting examples will serve to bring the main facts before us. 

 The first type is well illustrated by the egg of a Mollusc, Den- 



