us 



On the Sexual Cells of Millepora plicata. [Dec. 8, 



A second and larger spindle appears after the first polar globule is 

 thrown out, which in its turn discharges the second polar globule. 



The mature ova of M. plicata are very small ( T ^o mm - i* 1 diameter), 

 and contain no yolk globules or granules. 



After maturation the ova are impregnated. The heads of two or 

 three spermatozoa may be seen within a single ovum, the flagella re- 

 maining on the surface. 



After fertilisation the germinal vesicle is again apparent, and at a 

 later stage it is seen to contain a number of nucleoli. 



The germinal vesicle next fragments, the fragments being scattered 

 over that pole of the ovum which previously contained the germinal 

 vesicle, i.e., the pole nearest to the stalk. 



The fragments at a later stage travel towards the middle of the 

 ovum, where they form an equatorial zone. 



This equatorial zone of fragments divides into two parties, which 

 travel towards the poles. 



The fragments during these movements increase in size and in 

 number, and in the next stage observed they are scattered over the 

 whole ovum. 



This stage corresponds with the morula stage of other embryos, 

 the fragments of the original germinal vesicle being the nuclei of its 

 constituent cells. Yery faint markings in the substance of the 

 embryo indicate the outlines of the cells. 



The embryo next assumes the form of a solid blastosphere, in which 

 stage it migrates into the gastrozooid, and its subsequent history is lost 

 by its being discharged most probably by the mouth to the exterior. 



No trace of any medusa or medusiform gonophore or sporosac was 

 found either on the dactylozooids or gastrozooids containing ova or 

 embryos. 



The young male sexual cells or spermospores are at an early stage 

 distinguished from the young ova by their large nucleus containing 

 a coarse protoplasmic mesh work. 



The nucleus fragments and the fragments soon come to occupy the 

 whole spermospore. 



The spermospore is matured in the canals, and then migrates into the 

 basal endoderm of the dactylozooids, where its wall disappears, and a 

 colony of young spermoblasts pass into the cavity of the zooid. These 

 push out the wall of the zooid into the form of sporosacs, which they 

 occupy until they are mature. The sporosacs do not seem to be 

 formed before the advent of the spermoblasts. There is no spadix nor 

 any other indication of their being degenerate medusiform gono- 

 phores. In a very few cases they were found in the gastrozooids. 



The origin of the sexual cells in Millepora support the views of 

 the Hertwigs and Weismann that the ectoderm is the original seat of 

 the sexual cells in the Hydrozoa. 



