TrBULARIA SOLITARTA. 91 



ectoderm-cells continuous with a thick layer of ectoderm on 

 the outside of the gonophore. 



In the fibrous mass of ectoderm {e. a. n.) there can occa- 

 sionally be distinguished one or more small rounded cells (a.o.) ; 

 these can be regarded as abortive ova. 



Abortive female g'onophores of this nature are absolutely 

 sterile; but in the case of male gonophores the sterility may not 

 be complete. In the semi-abortive male gonophores a variable 

 number of ripe spermatozoa may be found scattered amongst 

 the fibrous or stringy ectoderm filling the umbrella cavity. 



This diiference in structure and the very frequent abortion 

 or semi-abortion of the terminal gonophore are remarkable ; 

 but before any safe conclusions can be arrived at a com- 

 parison must be made with other hydroids which bear gono- 

 phores on peduncles or blastostyles. 



(4) Formation of the actinula.— I have only made a 

 few observations in connection with the development of the 

 actinula. Among the ripe female gonophores one can find 

 cases in which the ova are apparently fusing together 

 (fig. 20, g. e. m.) . This may almost certainly be regarded as 

 the ingestion of ova by some one or more fertilised eg'gs. It 

 may be noticed that in fig. 20 the germinal epithelium has 

 slightly extruded itself through the opening of the umbrella, 

 and it is from this point, where fertilisation would readily 

 occur, that the ingestion of ova is taking place. 



Fig. 21 represents the youngest condition found. Here I 

 cannot say with certainty which is the fertilised nucleus ; 

 it is quite possible that the enlarged swollen nuclei s)ii and 

 sHc, may both be fertilised nuclei, as it is extremely probable 

 that a considerable number of the ova would be fertilised. 

 The reason for regarding these nuclei as distinct from the 

 others may be explained by reference to fig. 25. The in- 

 gested ova {i. 0.) can be seen, but the remarkably swollen 

 condition of the nucleus (fig. 21) is not apparent. The 

 germinal epithelium is ultimately fused into one or two 

 masses, according to whether one or two actinulfe will 

 ultimately be developed in the gonophore. Whether ferti- 



