1887.] 



On the Sexual Cells of Millepora plicata. 



245 



of the body of the JEuphoheria correspond exactly to the condition of 

 the young Julus. 



With regard to the double segments of Julus, Newport held that 

 each double segment corresponded to two segments originally distinct 

 which had fused together ; subsequent writers have held that each 

 double segment is a single segment which has developed a second 

 pair of legs. Now considering the double segments with regard to 

 the development as well as to the adult condition, we see that the 

 mesoblastic segmentation is double, so are the tracheal, the nervous, 

 and circulatory systems. The only part of these double segments 

 which is single is the dorsal plate with its stink glands which arise 

 as invaginations in it ; this dorsal plate being so enlarged as to form 

 a complete ring round the body of the adult. Looking at the 

 palaeontology, we find that in the Archipolypoda, a family including 

 the Archidesmidee, Euphoberidae and Archijulida3, the dorsal plate did 

 show distinct traces of a division. Therefore I think that each double 

 segment represents two complete segments, the dorsal plates of which 

 have fused together to make one plate. 



III. " On the Sexual Cells and the early Stages in the Develop- 

 , ment of Millepora plicata"' By SYDNEY J. HlCKSON, M.A. 

 Cantab., D.Sc. Lond., Fellow of Downing College, Cam- 

 bridge. Communicated by Professor M. Foster, Sec. R.S. 

 Received November 19, 1887. 



(Abstract.) 



The investigations were made upon several specimens of Millepora 

 plicata I found growing in abundance on the fringing reefs of Talisse 

 Island, N. Celebes. 



The young sexual cells, both male and female, are found in the ecto- 

 derm of the coenosarcal canals, between the dactylozooids and the 

 gastrozooids. 



At an early stage they leave the ectoderm, and by perforating the 

 mesoglcea take up a position in the endoderm. 



The ova at an early stage become stalked. The stalk of the ovum, 

 which is simply a modified pseudopodium, serves to keep the ovum 

 attached to the mesoglcea. 



The stalk may at times be completely withdrawn, and the ovum by 

 amoeboid movements migrate along the lumen of the canal to a more 

 favourable locality, where it becomes again attached to the mesoglcea 

 by a stalk. 



Before maturation the germinal vesicle disappears, and a spindle- 

 shaped body with longitudinal strias appears, which throws out the 

 first polar globule. 



