378 SYDNEY J. HICKSON. 



an ordinary zooid of the colony changed into a medusa after 

 the migration of spermospheres into its ectoderm, and subse- 

 quent development there. 



The evidence that supports this statement rests upon a 

 number of observed facts, that foi» convenience' sake may be 

 arranged under the following heads : 



1. The various stages in the transformation of the zooids 

 into medusae that can be observed in sections of the decalcified 

 corallum. 



2. The absence of any structure that can be compared to 

 the ectodermic invagination, called the entocodou or glocken- 

 kern, that characterises the early stages in the development of 

 the medusa of the Hydroidea. 



3. The position of the medusae in the colony. 



4. The presence of large nematocysts in the superficial ecto- 

 derm above the younger forms of medusae. 



1. The most important of these, and the only one upon which 

 much stress can be laid, is the first. The others afi'ord the 

 necessary confirmation. 



The earliest recognisable forms of the sperm mother-cells 

 are found in the canals in the immediate neighbourhood of 

 the zooids {Sperm. S^,., fig. 1). They migrate from this po- 

 sition into the ectoderm of the zooids, where they collect 

 together to form a spermarium. 



That the sperm mother-cells do actually migrate from the 

 germinal epithelium into the zooids seems to me to admit of 

 no doubt. The youngest stages of the germ-cells are never 

 found in any part of the zooids, and the youngest stages of 

 the zooids never bear either germ-cells or spermoblasts. 

 These two observations prove, firstly, that the germ-cells do 

 not arise in fully developed zooids ; and secondly, that new 

 zooids or medusae are not formed at the localities in the canals 

 where the germ-cells arise. They must, therefore, move from 

 the position where they are first developed to the position 

 they occupy in the zooid. 



In a few cases I have seen two or three spermospheres 

 {Sperm. S^., fig. 1), or aggregations of spermospheres, lying 



