Winter Eggs 265 



Statoblasts Perhaps the most specialized of over- 

 wintering bodies are those of the Bryozoans and fresh- 

 water sponges, known as statoblasts. These are little 

 masses of living cells invested with a tough and hard 

 and highly resistent outer coat. They are formed 

 within the flesh of the parent animal (as indicated for 

 Bryozoan in fig. 77 on p. 167), and are liberated at its 

 dissolution (as indicated for a sponge in the accompany- 

 ing figure) . They alone survive the winter. As noted 

 earlier in this chapter, their chitinous coats are often 

 expanded with air cavities to form efficient floats: 

 sometimes in Bryozoan statoblasts there is added to 

 this a series of hooks for securing distribution by ani- 

 mals (see fig. 150 on p. 247). Often in autumn at the 

 Cornell Biological Field Station collecting nets become 

 clogged with these hooked statoblasts. 



In the fresh-water sponges the walls of the statoblast 

 are stiffened with delicate and beautiful siliceous 

 spicules, and there is at one side a pore through which 

 the living cells find exit at the proper season. Since 

 marine sponges lack statoblasts, and some fresh-water 

 species do not have them, it is probable that they are 

 an adaptation of the life cycle to conditions imposed 

 by shoal and impermanent waters. 



Winter Eggs Another seasonal modification of the 

 life cycle is seen in the Rotifers and water-fleas. Here 

 there are produced two kinds of eggs; summer eggs 

 that develop quickly and winter eggs that hibernate. 

 The summer eggs for a long period produce females 

 only. They develop without fertilization. In both 

 these groups males are of very infrequent occurrence. 

 They appear at the end of the season. The last of the 

 line of parthenogenetic females produce eggs from which 

 hatch both males and females and the last crop of eggs 

 is fertilized. These are the over-wintering eggs. 



