DEVELOPMENT OF PLANULA IN A PLUMTTLARrAX PTYDlfOIO. 00 



the whole development. This phicental tissue arises as a 

 modification of a specialised portion of the ectoderm of the 

 blastostyle. This portion, as we have seen, forms a kind of 

 cap over the young ovum, and doubtless may be regarded as 

 representing the manubrial ectodei-m of a rudimentary gono- 

 phore which bears one egg. 



EXPLANATION OF PLATE VI, 



Illustrating Dr. Ernest Wnrren's paper, "On the Develop- 

 ment of the Planula in a certain Species of Plumularian 

 Hydroid." 



Explanatory References for Plate and Text-figures. 



B. Blastostyle. Bl. Blastoewl. h. G. Beginning of gouaugium. 

 C. Ccelenteron. c. j). Covering plate (" Deckenplatte '") of ectoderm of 

 gouangium. c. 0. Cavity above ovum, wliicli may be regarded as a 

 cavity between the manubrial ectoderm and endoderm of gonophore. 

 c. U. Cavity of umbrella, d. C. Diverticukim of cceleuteron representing 

 tlie endoderm of gonophore. d.jj. Dissolved perisarc. e. B. Ectoderm of 

 1 »lastostyle. e. G. Scattered ectoderm cells of gonophore. e. E. Outer 

 enveloping ectoderm derived from the scattered ectoderm cells of gono- 

 phore. c. I. External ectoderm layer of typical gonophore. e. I. U. Ecto- 

 derm layer of imibrella. ect.^. Ectoderm of planiila. end. Endoderm of 

 blastostyle and of gonophore. end^. Endoderm of planula. (j.e. Granular 

 endoderm. i. I. Inner or manubrial ectoderm layer of gonophore. M. 

 Morula. M. e. Manubrial ectoderm, n., n. Oblique nodes at base of 

 piunate portion of main-stem, n.^ i;., Transverse nodes at base of pinna. 

 n..2, n..2 Transverse nodes at the base of pinnule. 0. Ovum, o. I. Outer 

 layer of gonophore, equivalent to the layer of ectoderm lining the um- 

 brella-cavity, ojy. Opercuhim. j;. e. c. Posterior ectodermal space of 

 liydrocaulus. p. i. j). Proxiuial internode of pinnule. 2d- ^- Placental 

 tissue. S(j. C. Segmentation-cavity, s. j). Space between the external 

 ectoderm layer of gonophore and the ectoderm of the umbrella-cavity, 

 .s^).,, sj)... Spaces between perisarc of gonotheca and blastostyle. sp. c. 

 Spermatic cells in endoderm of hydi-ocanlus. t. Spermatic cells (testis) 

 of male gouaugium. t. p. Terminal jjortion of the cceleuteron of l)lasto- 

 style. V. m. Meuiln-ane around ovum. 



Fig. 1. — X -20. Longitudinal section throtigli the niain-steiu imme- 

 diately below a hydrotheca showing the ])egiuniiig of the gouaugium 



