PHYLUM PORIFERA SPONGES 159 



must be able to resist injurious conditions, or die. A large 

 part of the body may disintegrate when the water is foul or 

 when food fails; but if a few cells are left, they will grow into 

 a complete new sponge, just as a tree may send up a shoot 

 from its stump after being cut down. Sponges, then, 

 usually have little power of resistance but possess remark- 

 able ability to regenerate lost parts. Professor H. V. 

 Wilson has recently succeeded in growing complete sponges 

 from minute pieces which had been strained through a fine 

 sieve. The bodies of fresh-water sponges disintegrate in the 

 autumn, but certain groups of cells, the gemmules, form 

 resistant coats about themselves, remain dormant until 

 spring, and then grow into new sponges. 



Race Preservation. As has just been stated, sponges 

 have remarkable powers of regeneration. Many species are 

 even able to throw off parts of the body which grow into 

 complete individuals. In a large sponge it is often impos- 

 sible to determine how many individuals are present for the 

 canals are interwoven and cross-connected inextricably. 

 Sponges may be easily separated into parts without serious 

 injury and different individuals are as readily fused together. 

 All the methods of reproduction thus far described are 

 strictly asexual there is no fusion of sexual cells to form a 

 zygote, no maturation, fertilization, or cleavage. Sexual 

 reproduction does occur in sponges, however, and most 

 individuals start in the beginning from a fertilized egg. 



The ova and spermatozoa of sponges ripen during typical 

 maturation stages, as described in the last chapter, and 

 fertilization takes place as in other metazoans. Though 

 the cleavage stages are somewhat peculiar, they are much 

 like those in other many-celled animals. At- this point, 

 however, the resemblance between sponge development and 

 that of other Metazoa ceases, for no gastrula is formed. In 

 other words all metazoans, except sponges, pass through 

 maturation, fertilization, cleavage, and gastrula stages; the 

 sponges pass through the first three, but form no gastrula. 

 This is taken to mean that all many-celled animals except 



