10 EVOLUTION OF CELLULAR STRUCTURES. 



built of the primitive, .simple t3'pe of cells, as illustrated by the fila- 

 ment of the lower alga, the vegetative mycelium of the fungus, and 

 the thallus of the liverwort. The plant series would have culminated, 

 apparently, with the leafy axis of the moss if the basis of organiza- 

 tion had not l)een changed from the primary or simple type of cells to 

 the double or sexual tj^pe. 



In undifferentiated unicellular or equal-celled (isocytic) organisms 

 the successive generations of cells may be thought of as joined into a 

 network by an occasional conjugation. The cells at the knots of the 

 network are, as we know, double, being formed from the association 

 of two nuclei and the accompanying protoplasms. They are often 

 strikingly different from the remainder of the cellular fabric of 

 descent, and have been given special names, such as oospore, zygo- 

 spore, and resting spore. In the first or lowest category of sexual 

 organisms only one cell in each generation is double; there is only one 

 large bead at each node of the genealogical network. (See PI. I.) 

 A second type of organic structure was initiated when an organism 

 attained the art of forming two or more of these double cells by divi- 

 sion.'^* It is of such double cells that all the higher plants and animals 

 are built. The new type of organization was not merely supplemen- 

 tary to the old; it was a new biological invention, giving rise to a new 

 category of vitality, which not only outstripped the old type of struc- 

 tural organization, but even caused it to be abandoned and eliminated 

 as a worse than useless impediment. 



Organisms which were farthest ahead on the primitive basis have 

 fallen far behind since the new course of development was opened. 

 In such groups as the liverworts, mosses, and ferns the diversity of 

 the two types of organic structure is strikingly obvious, and has 

 received extensive study for years past under the name of " alterna- 

 tion of generations." Ample homologies have been found in the 

 highest groups of plants to show that the so-called alteration of gen- 

 erations was everywhere in ancestral condition, and that all have 

 followed essentially the same histor}'^ in having abandoned the simple 

 type of cell for the double as the basis of structural development. 



« The terminology followed in this paper presupposes for convenience the exist- 

 ence of the cellular type of organization common to most animals and plants. The 

 conclusions here reached apply with equal force, however, to organisms such as the 

 Infusoria among the protozoa, the Siphonocladiacepe among the algte, and the 

 Saprolegniales and Mucorales among the fungi, in all of which groups considera- 

 ble structural differentiation is attained without any division of the organism into 

 cells. Such forms as Caulerpa and Aceta1)ularia among the Siphonocladiacea^ reach 

 a considerable size and even show a well-marked differentiation into the analogue 

 of stem and leaf, rhizome and root, without the enormously expanded thallus being 

 divided up into cells at all, although very.numerous nuclei arise by subdivision and 

 are scattered througliout tlie cytoplasm. These nuclei could l)e double, in which 

 case such plants would be directly homologous to the double-celled organisms 

 described in the following pages. 



