334 INTRODUCTION TO CYTOLOGY 



contribute certain general characters, such as polarity, symmetry, and 

 general type of early development, which are the direct outgrowth of an 

 elaborate organization present in the egg cytoplasm. It is true that this 

 organization is the result of processes in which the nucleus cooperates 

 during the differentiation of the egg, and those who hold to the universal 

 applicability of the Mendelian interpretation would assume that the 

 type of organization must depend upon Mendelian factors carried in the 

 nucleus. However this may be, the fact remains that the two gametes 

 at the time of fertilization are not equal in hereditary potency, as 

 Conklin states. So far as the clearly Mendelian characters are con- 

 cerned, however, all evidence goes to show that they are precisely 

 equal. 



The direct inheritance of metidentical characters, such as the above 

 mentioned green plastid color in Pediastrum, and the indirect inheritance 

 of colony characters in the same form, afford other examples of hereditary 

 transmission otherwise than through the nucleus. With respect to 

 colony characters, Harper has shown in a striking manner, both in Hydro- 

 dictyon and Pediastrum, that the characteristic form and type of organiza- 

 tion assumed by the colony are the results of interactions between the 

 form, polarities, adhesiveness, surface tension, etc. of the free-swimming 

 swarm spores which aggregate to build it up. The swarm spore has an 

 individual organization of a particular type, but its capabilities show it to 

 be devoid of any arrangement of its protoplasmic parts corresponding 

 either to its future position in the colony or to the arrangement of 

 the cells in the colony as a whole. The character of the colony 

 thus depends upon the interactions of its component units and is 

 in no way represented in any one of them. Consequently it is held by 

 Harper that no system of spatially arranged factors in a special germ 

 plasm is required to account for the regular reappearance of such cell and 

 colony characters in these organisms, and that such facts must be reck- 

 oned with in attempting to explain heredity and development in terms 

 of the cell. 



By whatever means they are transmitted, it is evident that most 

 characters must be brought to expression through the activity of the cell 

 system as a whole, the process involving a long series of reactions in 

 which all or nearly all of the cell constituents play their parts. At the 

 present time little or nothing is known of the real nature of the " factor" 

 or of the manner in which it may influence the development of a character. 

 In general, then, we may say that the heritage bequeathed by an indi- 

 vidual to its offspring is in most organisms transmitted mainly through the 

 nucleus, since it is very largely upon this organ that the development or 

 non-development of particular characters in the organism depends; but 

 also that the development of the characters in the offspring, however these 



