REPRODUCTION 



171 



and the minute sponge emerges either by amoeboid movement or as a 

 result of growth. It soon assumes the form and appearance of a minute 

 sponge (Fig. 128) which by growth and external budding becomes the 

 large sponge as- it is found during the summer and fall. The gemmule 

 of the sponge is, therefore, not only a medium for 

 multiplying the number of individuals, but also a 

 means of dispersal of the species and a means of 

 surviving the vicissitudes of winter. 



The Bryozoa (moss animals) living in fresh 

 water also produce internal buds which when 

 enclosed in a dense protective covering are called 

 statoblasts. A statoblast of Cristatella mucedo is 

 shown in Fig. 129. In some lakes and ponds 

 where there are many Bryozoa statoblasts are 

 produced in such numbers that they float up on 

 the shore in quantities sufficient to blacken the 

 beach or cloud the water in which they float. 

 Statoblasts persist over winter and give rise in the 

 spring to new colonies. Since most Bryozoa are 

 attached organisms or have only limited loco- 

 motor powers the statoblasts serve as a means of 

 dispersal. 



Fission.^ — Fission is a more common reproduc- „ , „„ „, , , . 



'■ . Fig. 130. — Stylonychia 



tive method among Protozoa than budding. myUius dividing, c, con- 

 Fission differs from budding in that the body of tractile vacuole; iV, macro- 



'^ _ _ _ '' nucleus; n, micronuclei. 



the parent is about equally divided into two parts {From Sedgxnck, after stein. 

 which come to resemble the parent after a period CoxirtesyofMacmiiianCo.) 

 of growth and regeneration of missing parts. In fission, especially among 

 the Protozoa, the parent disappears as an individual and two new in- 

 dividuals take its place. The plane of fission may be longitudinal or 

 transverse. Transverse fission, that is, division perpen- 

 dicular to the long axis, is the more common. It is illus- 

 trated in Fig, 130 which shows Stylonychia in division. 

 Even in forms like Vorticella (Fig. 131), in which fission 

 appears to be longitudinal, the division is really trans- 

 verse, since the ciliated disk w^hich seems to be the end 

 of the cell is morphologically its ventral surface. 



Fission involves the nucleus as well as the cytoplasm. 

 Structures which extend across the plane of fission are 

 halved and the missing portion regenerated. Other structures go with 

 that portion in which they are located before fission, and corresponding 

 structures arise anew in the other portion. Thus in species with two 

 contractile vacuoles, one placed anteriorly, the other posteriorly, one 

 vacuole goes to each new individual and a second vacuole arises anew 



Fig. 13 1.— 

 Vorticella in divi- 

 sion. {From Kent 

 after Grecf.) 



