iH'i SYDNEY J. HICKSON. 



migration of spermospheres into its ectoderm, and subsequent 

 development there. 



The evidence that supports this statement rests upon a 

 number of observed facts, that for 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 entocodon 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 

 ectoderm 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 afford 

 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 position 

 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. >So., fig. 1), or aggregations of spermospheres, lying 

 separately in the ectoderm of the zooids, but in the majority 



