304: Dynamic Theory. 



formation of germ layers and the construction of a body. In a hermaph- 

 rodite worm the development of both the male and female glands ma} r 

 be traced from a single cell. This cell splits into two, each of which, 

 by growth and segmentation, becomes a sexual gland one male and 

 one female. When the animal reaches maturity these glands bear fruit 

 in the shape of sexual cells, and the reproduction of the animal follows 

 the union of a cell from one gland with a cell from the other. In the 

 development of the animal, as soon as the segmentation of the egg and 

 its division into cells has begun, one cell becomes set off to itself to 

 form the new sexual glands, while all the rest go on undergoing seg- 

 mentation and growth into the other parts of the body. If we compare 

 this process of reproduction with that of the mould Protomyxa, we see 

 that the part concerned especially in reproduction, imitates the round 

 taken by the mould. The ovum, of the worm answers to the cyst of the 

 mould. In the ovum, as in the cyst, the contents divide up into sub- 

 ordinate organisms. In both cases they become liberated. But here a 

 partial differentiation comes into view. Only two cells, arising from the 

 organization of the protoplasm of the ovum, persist in making the grand 

 circuit, while all the rest become enlisted in the formation of the or- 

 gans which go to make up the body of the animal, and so have no fur- 

 ther direct concern in reproduction. But these two exceptional cells do 

 essentially as the liberated tailed spores of the mould do, that is, they 

 form a plasmodium, or its equivalent, and this becomes encysted in the 

 form of the impregnated ovum ( cytula ), and the circle is complete. 

 There is, however, always more ceremony, preparation and delay in the 

 processes of sexual reproduction than the above comparison would seem 

 to imply, and the higher the animal the greater the complexity. Thus 

 it is only in a few simple animals that this special very early separation 

 of the cells devoted to reproduction from the rest, has been actually ob- 

 served. According to Geddes and Thomson, the fly, Chironomus, is 

 the best example, while it has also been observed in other insects; in 

 the worm Sagitta, in leeches, in nematode or thread worms, in some 

 crustaceans, in the water-flea Moina, in some spiders, and in other an- 

 imals. In all cases there is a pause in the development of these sepa- 

 rated cells ; that is, they do not immediately proceed with the function 

 of reproduction, but appear to wait for the body cells to gain a degree 

 of development first. These last appear to monopolize the plastic ma- 

 terial in their own development until they approach a certain degree of 

 completion, when there is a let up in their demand and the material and 

 energy go toward the support of reproduction. The duration of the in- 

 terval between the segmentation of a c} 7 tula and the formation of a 

 daughter ovum varies greatly in different animals. Among mammals 

 it ranges from about two months to twelve years or more, depending on 



