448 R. M. PACE. 



obvious I'elation to tlie process of yolk formation. In this 

 form oil globules are present in very young eggs, both in the 

 yolk nucleus itself and also scattered throughout the sur- 

 rounding protoplasm ; and, although they are certainly present 

 in increasing numbers as the yolk develops, they do not 

 disappear with the completion of yolk formation, but persist 

 throughout larval life. So far, also, in Flustrella it has not 

 been possible to confirm Bambeke's views as to the importance 

 of the nucleus as a factor in yolk formation. The irregular 

 shape of the nuclear membrane, which is sometimes observed 

 in the early stages of yolk formation in the egg of Flustrella 

 (PI. 23, fig. 24), may possibly indicate amoeboid movements ; 

 but these have no apparent reference to the distribution of 

 the oil drops, which collect chiefly towards the periphery of 

 the egg rather than in the immediate neighbourhood of the 

 nucleus; and neither the oil globules nor the yolk spherules 

 show the radial arrangement which is described by Bambeke. 

 Crampton (9), in his paper on the early history of the 

 Ascidian egg, described a yolk nucleus of similar type in the 

 egg of Molgula manhattensis ; and he attempted to 

 determine its chemical nature by differential staining. The 

 general account given by Crampton of the history of the yolk 

 nucleus in Molgula agrees with the above description of 

 this structure in Flustrella. Crampton, among other 

 stains, made use of Heidenliain'siron-hfematoxylin, and states 

 that this stain is taken up by the chromatin of the germinal 

 vesicle, but that it has no effect on the yolk nucleus ; he also 

 mentions the difficulty experienced in washing out this stain 

 from theyolk after yolk formation, without at the same time 

 decolourising nuclear structures. In the case of the egg of 

 Flustrella, iron-hsematoxylin stains both the yolk nucleus 

 and the chromatin elements of the nucleus with equal inten- 

 sity, and a similar difficulty with regard to the washing of 

 eggs containing yolk has been experienced. PI. 23, fig. 25, 

 shows a section of an egg in which, even after prolonged 

 washing with iron-alum, the centres of many yolk spherules 

 remained darkly stained. Crampton's researches led him to 



