340 JAMES F. GEMMILL 
A. Agassiz (1) described the young hydroid reared from the 
eggs of Melicertum, but this hydroid has not up to the 
present, been recorded in nature from the American coasts. 
Development of Eggs of Melicertidium. 
Ripe examples appeared in the tow-nettings at Millport 
towards the end of June 1918. By keeping specimens in 
aquaria in the Research Fellowship Laboratory at Glasgow 
University I obtained numbers of fertilized eggs. These are 
small (0-08 mm. in diameter), homogeneous-looking, faintly 
yellowish in tinge, and with delicate closely-adherent mem- 
brane. They are ripe before extrusion and pass outwards 
through the mouth, as also do the spermatozoa in the males. 
No membrane of fertilization is formed. Segmentation is total 
and equal (figs. 1-5), the two-celled stage beginning with 
a notch or groove on one side of the egg. A blastocoele cavity 
is recognizable even at the eight- or sixteen-celled stage. 
Karly blastulae are irregular in outline, the blastula wall being 
a single layer, but exhibiting folds and inpocketings which soon 
straighten out and do not seem to have any subsequent forma- 
tive importance (figs. 6 and 7). ‘The larva now becomes pear- 
shaped, and, having acquired cilia, progresses with the blunt 
end in front and rotates in the solar direction as viewed from 
the blunt end (figs. 8 and 9). At this stage the endoderm 
arises by inward budding from the blastula wall (figs. 8, 9, 10). 
The budding occurs first near the pomted end, and then all 
round, gradually filling up the blastocoele cavity, the last part 
of this cavity to be filled beimg at the blunt end (fig. 11). The 
endoderm cells are rounded, shghtly granular, and less trans- 
parent than the ectoderm. The planula now elongates, becom- 
ing almost worm-like, and swims vigorously through the water 
at any depth. Later it seeks the bottom and becomes attached. 
The mode of attachment presents certain peculiarities which 
I hope to elucidate later. The free end becomes swollen and 
rudiments of the first tentacles appear (fig. 12). Figs. 12-14 
illustrate four-tentacled and eight-tentacled stages. Both 
show a delicate perisare covering hydrorhiza and hydrocaulus, 
