On the Rhizocephalan Genus Thompsonia, etc. 



19 



cells (text-figure 9A). 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 9s). 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 



10 



an. 



FIG. 10. "Pro-Cypris" larva of Thompsonia from Thalamita in egg-membrane. X375. 

 FIG. 11. Cypris larva of Thompsonia from Synalpheus. The dotted area in the ventral part of 

 the body contains reserve materials, an., antenna. X470. 



within the egg membrane. While, moreover, the egg of Sacculina is 

 comparatively large, 600 to 800 fj. in diameter, that of Thompsonia 

 measures only about 34 //. 



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 D, E, F) thus takes place by 

 epibole. 



Beyond the gastrula I have not been able to trace development. The 

 next stage observed is comparatively far advanced, showing a single 

 pair of rudimentary processes, probably representing the antennae. I 



