3i8 



ANIMAL DEVELOPMENT 



Fig. 842. — A Marine Worm {Myriauida) 

 giving rise to a chain of individuals by trans- 

 verse division. A, The parent worm; b-g, the 

 new individuals, of which b is the oldest and 

 G the youngest 



enable this development to take place. But many of the lower 

 Metazoa do not entirely rely upon eggs for the perpetuation of 

 their kind, but also give rise to other individuals by means of 



what is termed vegetative propagation, 

 with which the egg-producing cells 

 have nothing to do. There are, for 

 example, some marine worms (fig. 

 842) in which the body becomes 

 transversely divided into a number 

 of sections, each of which ultimately 

 breaks off and becomes a fresh worm. 

 This phenomenon is much on a par 

 with what is seen in many Flowering 

 Plants. Here the production of seeds 

 is related to egg-propagation, but this 

 is frequently supplemented by one or 

 more vegetative methods of multipli- 

 cation. In Begonias, for example, 

 buds sometimes o-row out at the edaes 

 of the leaves, which, after developing into little plants, fall off 

 and take root in the ground. 



Vegetative Propagation in Animalcules (Protozoa). — With 

 rare exceptions the body of an Animalcule consists of but one 



cell, and there is therefore no question of 

 egg-production, propagation being effected 

 vegetatively. Three kinds of this are 

 recognized: Splitting (Fission), Budding 

 (Gemmation), and Spore- Formation. All 

 are cases of cell-division. 



Splitting [Fission). — A simple case of 

 this is afforded by the Proteus Animalcule 

 {Amceba, fig. 843). Division of the body 

 is initiated in the nucleus, which becomes 

 elongated and dumb - bell - shaped, after 

 which the surrounding protoplasm gradu- 

 ally constricts into two, and ultimately 

 there is complete separation, with the 

 result that two Animalcules take the place of the original one. 

 The parent Amoeba is thus merged in its offspring, or at any 



Fig. 843. — Stages in the Fission 

 of a Proteus Animalcule [Ania^ba], 

 much enlarged. Read from left 

 to right, beginning at the top. 

 The nucleus is represented black 

 with a clear margin 



rate has ceased to exist as a single individual. 



According to 



o 



