REPRODUCTION AND DEVELOPMENT 



563 



tudinal division into a great number of cells. 

 As the cells divide, they bulge inward to 

 form a pocket, the new daughter colony, 

 without fusing with a gamete. 



The sexual method of reproduction may 

 be observed in colonies which contain as 

 many as 50 large nonflagellated cells. Some 

 of these grow larger and may be recognized 

 as female gametes (eggs); others produce 

 by longitudinal division a flat plate contain- 

 ing many spindle-shaped male gametes 

 (sperms). One sperm fuses with each egg. 

 The zygote thus formed secretes a surround- 

 ing wall and remains in this condition for 

 some time. Eventually the zygote breaks out 

 of the wall and produces a new colony by 

 multiple division. However, the zygotes are 

 retained in the parent colony until the 

 somatic cells of the latter die and disin- 

 tegrate. 



In Volvox, true somatic cells are encoun- 

 tered for the first time, that is, cells which 

 are unable to reproduce the colony. In the 

 other forms described, every cell has the 

 capacity of reproducing the whole. Volvox 

 also contains true germ cells, that is, cells 

 that are set aside for purposes of reproduc- 

 tion. Furthermore, a clear case of natural 

 death occurs in the somatic cells when they 

 fall to the bottom of the pond and disin- 

 tegrate. The situation in Volvox is similar 

 to that in higher animals in which the body 

 consists of many cells which may be sepa- 

 rated into somatic cells and germ cells. The 

 latter are either male or female. In most 

 cases the fusion of a male cell with a female 

 cell is necessary before a new organism can 

 be reproduced. At any rate, some of these 

 germ cells maintain the continuation of the 

 species by producing new individuals, while 

 the somatic cells perish when the organism 

 dies. 



Looking back over this series, it will be 

 seen that there is a trend from a small sim- 

 ple aggregation of cells to a large and highly 

 complex colony composed of cells that show 

 considerable division of labor. Also, there is 



the origin and progressive development of 

 sexual reproduction. 



Germ plasm and somatoplasm 



As already noted, when the germ cells 

 become mature, they separate from the 

 body, giving rise to a new generation, 

 whereas the somatic cells die. This has given 

 rise to the idea that the somatic cells con- 

 stitute a sort of vehicle for the transporta- 

 tion of the germ cells. From this idea has 

 developed the theory of the continuity of 

 germ plasm, according to which the germ 

 plasm is a self-perpetuating type of proto- 

 plasm, which gives rise to both the proto- 

 plasm of the gametes ( germ plasm ) and to 

 the protoplasm of the somatic cells ( somato- 

 plasm) in each generation. The somato- 

 plasm dies periodically, but the germ plasm 

 is potentially immortal. 



Metagenesis 



Metagenesis is not common in the animal 

 kingdom but occurs frequently in certain 

 groups. Metagenesis, or alternation of an 

 asexual generation with a sexual generation, 

 is not uncommon among the coelenterates. 

 For example, the asexual generation of the 

 hydroid Obelia (Fig. 57) reproduces by 

 budding; some of these buds are small jelly- 

 fish which produce either eggs or spermato- 

 zoa; the fertilized eggs develop into asexually 

 reproducing hydroids. Gametes are produced 

 by the jellyfish Aurellia (Fig. 60), and 

 the fertilized eggs develop into a very 

 small hydroid stage which produces jelly- 

 fishes by budding. 



Oogenesis and spermatogenesis 



The gametes that are set aside during em- 

 bryonic development become either the fe- 

 male type, oogonia, or the male type, sper- 

 matogonia. In certain species, for example 

 in Miastor, a definite number of germ cells 



