588 GRIFFIN. [Vol. XV. 



most rapid. In all these stages, up to eggs 47/4 in diameter, 

 the cytoplasm shows an extremely dense reticulum with no 

 yolk-spheres. As growth continues, these appear in increasing 

 number in the meshes. It would, therefore, seem that growth 

 results as a distention of the reticulum by deposition of deuto- 

 plasm-spheres in the meshes, as in Ckaetopterus (Mead, '97). 



In mode of growth, the ova of Thalasscnta present some 

 interesting resemblances to those of the allied genus Bottellia, 

 though differing in several important particulars. In Bonellia, 

 as Vejdovsky ('vs) has most clearly shown, the eggs arise from 

 a peritoneal fold, and breaking away in clusters, undergo their 

 subsequent development floating freely within the body cavity. 

 But one (the central) cell of each cluster develops into an ovum, 

 the remainder functioning as nutritive and follicle cells. Vej- 

 dovsky shows that the egg develops at the expense of the 

 nutritive cells, where the follicle cells become flattened and 

 give rise to an outer egg membrane. In Thalassema only the 

 peripheral cells develop into ova, though the precise fate of the 

 more central ones is difficult to determine. Differential stain- 

 ing alone yields no evidence that they contribute in any way 

 to the growth of the eggs. With Auerbach's acid-fuchsin 

 methyl-green stain ('96), the reactions of the nuclei and cyto- 

 plasm are characteristic and opposite, the chromosomes and 

 nucleoli taking a pure green, and the cytoplasm and nuclear 

 reticulum a pure red. This yields equally little evidence that 

 any of the chromatin is expelled from the nucleus to give rise to 

 yolk material, as described by numerous observers (Blochmann, 

 V. Bambeke, Balbiani, Calkins, and others). The cell outlines, 

 especially those of the growing egg, are sharp, and the single 

 hyaline membrane is distinct. There is here no follicular or 

 cellular membrane, as in Bonellia. 



Nucleus. — It is important to note that as the egg increases 

 in size, the nucleus takes up an eccentric position on the side 

 of the free surface of the egg (PI. XXXI, Figs. 1-3). Thus 

 the early, if not the ultimate polarity of the egg is to be 

 referred to its position in the cluster, the attached side becom- 

 ing the vegetative, and the free end the animal pole, as in Cyclas 

 (Stauffacher, '93) and other forms. The bearing of this will 



