On the Rhizocephalan Genus Thompsonia, etc. 19 
cells (text-figure 94). These latter, when teased out from the living 
parasite, take up amoeboid forms. In the oocyte stage the germ cells 
increase in size and yolk globules accumulate within them. In the 
internal buds this substance is confined to the external layer, but in the 
external sacs it occurs within both the visceral mass and mantle. The 
interstitial cells contain it and very possibly transfer it as required to 
the developing eggs. 
When the oocyte attains its full size the nucleus lines on one side with 
the yolk granules occupying the largest part (text-figure 98). They 
vary in number and are very unequal in size, but there are generally 
three or four large ones. The egg is very unlike that of Sacculina with 
its numerous equal-sized globules, which seem to occupy the whole space 
Fig. 10.—‘‘Pro-Cypris” larva of Thompsonia from Thalamita in egg-membrane. 375. 
Fig. 11.—Cypris larva of Thompsonia from Synalpheus. The dotted area in the ventral part of 
the body contains reserve materials. an., antenna. 470. 
within the egg membrane. While, moreover, the egg of Sacculina is 
comparatively large, 600 to 800 uw in diameter, that of Thompsonia 
measures only about 34 uw. 
Segmentation commences unequally, a micromere and a macromere 
with all the yolk being formed. Both micromere and macromere take 
part in the second (text-figure 9c) and probably also in the third division, 
but after this the yolk-containing cells cease to divide. The continued 
division of those without yolk leads to their rapid extension over the 
yolked cells and gastrulation (text-figure 9 p, £, F) thus takes place by 
epibole. 
Beyond the gastrula I have not been abletotrace development. The 
next stage observed is comparatively far advanced, showing a single 
pair of rudimentary processes, probably representing the antenne. I 
