There are a number of other forms of asexual reproduction, or the “vegetative type” (Abbott’s term, which includes fission, budding, polysporogonia and simple spore formation). Budding (as in yeast) and spore formation are familiar to us in plants. Such forms are too distant from man, in structure and function, for profitable direct comparison. Especially is this true with respect to sex, which they do not possess.
Parthenogenesis includes very diverse and anomalous cases. The term signifies the ability of females to reproduce in such species for one or a number of generations without males. Many forms of this class (or more strictly, these classes) have apparently become specialized or degenerated, having once been more truly sexual. Parthenogenesis (division and development of an egg without the agency of male sperm) has been brought about artificially by Jacques Loeb in species as complicated as frogs.[1,2] All the frogs produced were males, so that the race (of frogs) could not even be theoretically carried on by that method.
The origin of sexual reproduction in animals must have been something as follows: The first method of reproduction was by a simple division of the unicellular organism to form two new individuals. At times, a fusion of two independent individuals occurred. This was known as conjugation, and is seen among Paramecia and some other species to-day. Its value is probably a reinvigoration of the vitality of the individual. Next there was probably a tendency for the organism to break up into many parts which subsequently united with each other. Gradually some of these uniting cells came to contain more food material than the others. As a result of their increased size, they possessed less power of motion than the others, and in time lost their cilia (or flagella) entirely and were brought into contact with the smaller cells only by the motion of the latter. Finally, in colonial forms, most of the cells in the colony ceased to have any share in reproduction, that function being relegated to the activities of a few cells which broke away and united with others similarly adrift. These cells functioning for reproduction continued to differentiate more and more, until large ova and small, motile spermtozoa were definitely developed.
The clearest evidences as to the stages in the evolution of sexual reproduction is found in the plant world among the green algae. In the lower orders of one-celled algae, reproduction takes place by simple cell division. In some families, this simple division results in the production of several new individuals instead of only two from each parent cell. A slightly different condition is found in those orders where the numerous cells thus produced by simple division of the parent organism unite in pairs to produce new individuals after a brief independent existence of their own. These free-swimming cells, which apparently are formed only to reunite with each other, are called