368 COOK 



the higher types of life is itself a compact system or colony of 

 cellular organisms, and that these compound units are not only 

 different as to the aggregate cell-individuals, but there are dif- 

 ferent kinds of cellular organizations. Not only does endless 

 diversity exist among the unicellular or single-celled types of 

 life ; there are also different manners and degrees of cell-asso- 

 ciation to make up the multicellular types. If the cells of the 

 colony-individuals are alike, the organism is called isocytic, if 

 unlike heterocytic. 



If the cells which associate have no separating cell-walls the 

 organism may be described as plasmodial, as in the Myxomy- 

 cetes and in such alga? as Caulerfia and Acetabularia. If the 

 cells have the form of long slender filaments the organism is 

 described as hyphal, as in the fungi ; if built of definite cell 

 blocks it is called cellular, in the strict sense. The fourth or 

 highest type, found in the animals, combines the other three. 

 Some cells remain quite free and unattached, like the red and 

 white blood corpuscles ; some tissues are still plasmodial, others 

 hyphal, while still others, and these in the majority, have 

 definite cellular structure. 



Finally, the colony-individuals differ in being built of cells 

 which are not conjugating (agamic cell-structures) or of those 

 which are in conjugation (conjugate cell-structures). Of the 

 latter there are two types, the first is that shown by the higher 

 fungi which build colony-individuals of binucleate cells, formed 

 before the nuclei have fused in conjugation (apaulogamic cell- 

 structures). The second type of conjugate structure is that of 

 the higher plants and animals whose bodies are built up of cells 

 with the nuclei fused, but with a double number of chromosomes 

 (paragamic cell-structures). 



These facts are capable of a very definite graphic represen- 

 tation in our ideal longitudinal sections of specific networks of 

 descent. Double-celled structures are the conjugate product of 

 two lines of descent and their existence is to be shown in our 

 diagram by double, closely parallel lines. The network which 

 represents the method of descent of intermediate groups, such as 

 thearchegoniate plants (liverworts, mosses and ferns), may show 

 single and double lines in almost equal proportions. Primitive 



