DE VEL OPMENT. 1 27 



regenerate the whole a fact which illustrates the relatively 

 undifferentiated state of the sponge body. It is possible 

 that fission may sometimes occur naturally. 



The frequent budding is merely a kind of continuous 

 growth, but when buds are set adrift, as sometimes happens, 

 we have discontinuous growth or asexual reproduction. 



In the fresh-water Spongillid^e there is a peculiar mode of reproduc- 

 tion by statoblasts or gemmules. A number of mesoglceal cells occur 

 in a clump, some forming an internal mass, others a complex protective 

 capsule, with capstan-like spicules, known as amphidiscs. According 

 to W. Marshall, the life history is as follows : In autumn the sponge 

 suffers from the cold and the scarcity of food, and dies away. But 

 throughout the moribund parent gemmules are formed. These survive 

 the winter, and in April or May they float away from the dead parent, 

 and develop into new sponges. Some become short-lived males, others 

 more stable females. The ova produced by the latter, and fertilised 

 by spermatozoa from the former, develop into a summer generation of 

 sponges, which, in turn, die away in autumn, and give rise to gemmules. 

 The life history thus illustrates what is called alternation of generations 

 (see p. 57)- Interpreted from a utilitarian point of view, the formation 

 of gemmules is a life-saving expedient. As Professor Sollas says, " the 

 gemmules serve primarily a protective purpose, ensuring the persistence 

 of the race, while as a secondary function they serve for dispersal." 



All sponges produce sex cells, which seem to arise from 

 amoeboid mesogkea cells retaining an embryonic character. 

 In the case of the ovum, the amoeboid cell increases in size, 

 and passes into a resting stage ; in the case of the male 

 elements, the amoeboid cell divides into a spherical cluster 

 of numerous minute spermatozoa. The similar origin of 

 the ova and spermatozoa is of interest. Most sponges are 

 unisexual, but many are hermaphrodite. In the latter case, 

 however, either the production of ova or the production of 

 spermatozoa usually preponderates, probably in dependence 

 on nutritive conditions. 



Development. It is not surprising to find that there is 

 great variety of development in the lowest class of Metazoa ; 

 it seems almost as if numerous experiments had been made, 

 none attended with progressive success. 



The minute ovum, without any protective membrane, 

 usually lies near one of the canals, and is fertilised by a 

 spermatozoon borne to it by the water. It exhibits a certain 

 power of migration, as in some Hydroids. Previous to 



