DEVELOPMENT OF EGGS IN MANY-CELLED ANIMALS 341 



die, these dormant embryos enable the species to tide over an 

 unfavourable season, answering the same purpose as the winter 

 buds of freshwater moss-polypes (p. 330) or the seeds of annual 

 plants. During the following spring development is renewed, the 

 changes which take place converting the solid Blastula (morula) 

 into a two-layered Gastrula, possessing a central digestive cavity. 

 The protective coat is ruptured, the embryo elongates, a mouth 

 and tentacles are developed, and we have now a young Hydra, 

 which feeds actively and grows into an adult. 



The two-layered stage in development, i.e. the Gastrula, is of 

 great interest, and in one form or other represents a stage in the 

 life-history of the members of a great many groups of animals. 

 In its most typical form a mouth is present from the first, and 

 often is used as such, food passing through it into the central 

 digestive cavity (archenteron, i.e. primitive gut). Some specialists, 

 indeed, would not apply the term Gastrula to a two-layered embryo 

 devoid of mouth. We have seen how difficult it is to decide which 

 of the various kinds of Blastula are to be looked upon as repre- 

 senting an early ancestral stage. The Gastrula, however, probably 

 repeats a somewhat later phase in the evolutionary history of 

 Metazoa with some approach to accuracy. The Ccelenterates 

 remain throughout life permanent and specialized Gastrulae. And 

 according to the Gastraea theory of Haeckel all the higher groups 

 of animals are entitled to place some variety or other of the 

 Gastrula in their picture-gallery of ancestors. Other authorities, 

 however, are by no means unanimous on this point, though there 

 seems to be a good deal of truth in Haeckel's view, the weak 

 point in which lies in the assumption that the stage preceding the 

 Gastrula was a hollow sphere, which attained the double-layered 

 stage by in-pushing, much as an air-containing india-rubber ball, 

 when collapsed, becomes a double-walled cup. Such a process 

 certainly does not take place in the development of Hydra, but is 

 true for a number of other animals, as we shall presently see. 



Development of a Simple Calcareous Sponge. The Sponges 

 (Porifera) constitute a very problematic group of animals, of which 

 the relationship to other forms is obscure. They are sometimes 

 regarded as a branch of Ccelenterates, but it is more likely that 

 they are a distinct group or phylum which has taken independent 

 origin from Protozoan-like ancestors. It does not fall within the 

 province of this work to discuss so difficult a technical problem, 



