0» DEVELOPMENT IN PLUMULARIA. 



blastostyle bearing' one egg at its extremity. There is a large 

 rounded operculum at the distal surface. The female gono- 

 theca is markedly flattened in a plane at right angles to the 

 antero-posterior median vertical plane passing through the 

 main-axis. It bears near its base a pair of lateral nematophores. 



We have now to describe in greater detail the orig^in of the 

 e^g, the growth of the female gonotheca and es;s, and the develop- 

 ment of the embryo. 



The amount of the material available was fairly consider- 

 able. The specimens were fixed carefully in warm, corrosive 

 sublimate and acetic acid solution, and stained with Delafield 

 Haemotoxylin followed by Orange. Some six pinnate stems 

 with female gonophores were sectioned in different planes, and 

 owing to the fact that the gonophores in succeeding internodes 

 of the main stem were in different conditions of growth, all 

 stag'es of development would seem to be present. 



Notwithstanding this fact, the youngest ovum that could be 

 identified was already in the endoderm, and was situated just 

 below the sub-calycine nematophore. Presumably, according to 

 August ^^'eismann, and from analogv with observations on other 

 hydroids, the ova first arose in the ectoderm and then migrated 

 into the endoderm, but fhev were not definitelv located in the 

 outer la\ er, although carefullv searched for. The ovimi is small, 

 measuring about 14/^ in diameter, and is surrounded by 

 ordinary endoderm cells. The presence of the ovum causes a 

 slight swelling projecting into the lumen of the internode. 



The ectoderm immediately above the area where the ovum is 

 imbedded in the endoderm, earlv becomes slightlv modified in 

 that the cells are more :olumnar and somewhat taller than the 

 crdinarv ectoderm cells. The chitinous perisarc situated just 

 above is become markedly thinner, apparcntlv through being dis- 

 .-olved bv the subjacent ectoderm. 



In the next stage the plate of ectoderm grows out into an 

 ovoid swelling, which is covered b\' a verv delicate layer of 

 perisarc continuous with tlie general perisarc. The endoderm 

 follows the ectoderm, and the whole structure is the beginning 

 of the female gonotheca. The portion of endoderm carrying 

 the ovum passes into the gonotheca at the time of its formation. 

 and 1)ecomes located on one side of it. The diameter of the e^^ 

 has increased to 17 fi 



Ey this time the mesoglea between the ectoderm and the 

 area of endoderm in which the ovum occurs becomes thin and 

 evanescent. At the same time the ovum graduallv becomes 

 separated from the endoderm, and the ectoderm around becomes 

 modified and forms a kind of cap over it. This cap mav be 

 legarded as rei)resenting a rudinientarv gonophore. 



The distal i:)ortion of ectoderm of the voung gor.otheca 

 consists of columnar cells with pear-shaped, coarsely granular 

 cells wedged between. This is the beginnin.g of the " covering 

 plate." The endoderm with the c^ielenteron expands distally and 



