QO DEVELOPMENT IN PLUMULARIA. 



In the next stage considerable changes have taken place iii 

 that the whole of the " covering plate " of ectoderm and also 

 the upper horizontal limbs of the T-shaped endodcrm have 

 entirely disappeared. The out-pushing of endoderm has grown 

 upwards and has pushed the egg, together with the surrounding 

 cells derived from the breaking up of the columnar ectoderm 

 cells of the rudimentary gonophore, into a more or less central 

 position in close contact with the operculum. The membrane 

 around the ovum becomes less distinct. 



Sul)sequently the ovum becomes quite central at the apex 

 of the gonotheca. and is closelv surrounded bv the tissue derived 

 from the rudimentary gonophore. The endoderm is now in the 

 form of two symmetricallv i<laced lateral lobes, one being formed 

 from the endodermal out-pu.shing and the other is the end of the 

 main axis of the endoderm of the blastostyle. The egg is seated 

 symmetricallv between the two lobes. The ovum ■segments an'! 

 a rounded cluster of about t6 iooselv attached blastomeres i-- 

 formed. The endoderm immediatelv under the voung embrvo 

 consists of taller and more gramdar cells than those of the rest 

 of the endoderm of the blastostvle. 



The number of blastomeres increases, and they become 

 smaller. A segmentation cavity soon appears, and tliis is not 

 central l)ut occurs nearer to the apex of the gonotheca, so that 

 below the embryo is two or three cells in thickness, while above, 

 it is mostly only one cell thick. 



The embryo is now being obviouslv supplied with nouri^li- 

 ment bv the cells in which it is embedded. The diameter of the 

 embrvo is 2^ times that of the original ovum, which was never 

 swollen with ^•olk. The embrvo continues to expand, and the 

 segmentation cavity increases in size. A differentiation in the 

 cells of the embryo now becomes evident in the lower side, where 

 an inner layer of flattened cells can be seen. This laver lines 

 the segmentation cavitv on the lower half only. Tt would appear 

 to arise rather by differentiation than bv delamination. Tlie 

 cells around the embryo constitute a kind of placental feeding 

 tissue, and the tissue between the embrvo and the endoderm 

 appears to be the most active portion in the transfer of nourish- 

 ment. Here some of the cells are rounded or stellate, and they 

 were doid^tless amreboid. 



The embrvo begins to sink further into the gonotheca, and 

 Ijecomes more widelv separated from the apex. The embryo has 

 assumed a pear-shaped outline, and the two lobes of endoderm 

 grow upwards around it, followed bv the ectoderm. The cells 

 of this ectoderm are verv elongated and extended to the walls of 

 the gonotheca. The ectoderm between the embrvo and the lobes 

 of endoderm retains the same characters as before, and doubtless 

 it carries nourishment from the endoderm to the embryo. The 

 inner flat layer of cells of the embrvo. which constitutes the be- 

 ginming of the endoderm. is still confined to the lower half or 

 two-thirds of the embryo. 



