DEVELOPMENT OF PLANCLA IX A PLUMULARIAN HYDKOID. 93 



is a narrow space (figs. 3 and 4, .s^^^-)- ^^^'^^ long diameter of 

 the ovum has now increased to about 20 ju. 



The gonangium continues to expand, and the ovum grows 

 slightly and reaches a diameter of about 2S fi, but there is no 

 obvious yolk passed into it. The covering cap of ectoderm 

 of two layers also grows, the outer layer (fig. 5, o. I.) becomes 

 thinner, and the inner {i. I.) thicker. A delicate mesogloea is 

 formed between the ectodermal cap and the endoderm. The 

 endoderm cells on Avhich the ovum is seated are granular and 

 in an active physiological condition ; they stain more readily 

 than the rest of the endoderm. 



The gonangium expands further, and the distal plate of tall 

 columnar ectoderm cells (fig. 5, c. p.) begins to secrete a 

 thicker layer of perisarc ; this is the beginning of the 

 operculum. The blastostyle, except in the region of the 

 operculum, has become quite separated from the chitinous 

 gonotheca, and lies in the mid-axis. The ectodeimi lining 

 the umbrella-cavity of the gonophore, consisting of the two 

 layers, becomes separated from the ovum, so that a very 

 definite cavity occurs above the egg (fig. 6, c. 0.). This 

 cavity may be regarded as a space in the mesogloea of the 

 gonophore. The umbrella-cavity (c. U.) of the gonophore 

 is very narrow and almost obliterated. 



The ovum is probably fertilised just, before this stage, and, 

 presumably after such fertilisation, it appears to secrete a 

 kind of vitelline membrane (figs. 6, 7, 8, v. in.) which is 

 thicker on the outer than on the inner side. It is possible, 

 however, that this membrane is really of the nature of 

 mesogloea, and is not actually formed by the egg. The endo- 

 derm below the ovum becomes slightly pushed outwards into a 

 blunt process (fig. 8, d. C.) consisting of narrow granular 

 cells, and the egg is seated on this out-pushing, which may 

 be regarded as the spadix of the reduced gonophore. The 

 ovum now attains its maximum diameter of about 34 /./. 



In text-fig. 4 a comparison is made between a typical 

 gonophore (A) and the highly modified gonophore (B) 

 carried by the blastostyle of the present species. 



