MUSEUM OF COMPAKATIVE ZOOLOGY. 425 



observations on the maturation of the ovum of Polystomum integerri- 

 , mum. A thickening of the yolk forces the germinative vesicle to one 

 side of the egg, when its section becomes more or less crescentic. The 

 vesicle disappears, leaving behind only a homogeneous light space and 

 faint indications of radiation. The spherical form of the yolk is ex- 

 changed for a more flattened one. Two nuclear structures appear near 

 tlie surface after the , egg has resumed its spherical condition, and unite in 

 the middle of the yolk to form a nucleus which soon disappears. 



Another case of misinterpretation of the pronuclei similar to Salen- 

 sky's is that of Barrois ('76, p. 16, PI. XII. Fig. 2), who says, " Certain 

 eggs [before segmentation] present two nuclei ; they are the nuclei of 

 the first two spheres of segmentation." 



Ed. van Beneden ('76*^, p. 49) thinks the germs of the infusoriform 

 embryos of Dicyema do not lose their nuclei, as eggs do their germina- 

 tive vesicles, but that the nucleus divides, and thus gives rise to the 

 nuclei of the first two embryonic cells. 



Bobretzky's ('76, pp. 97, 98, 100) observations on the stages em- 

 braced under maturation are very limited. At a point on the surface 

 of the freshly laid egg of Nassa mutabilis a small whitish spot is to be 

 seen. Nothing is said about the way the polar globules are formed ; 

 but there are two recognizable with each egg soon after the extrusion of 

 the latter, and they are joined to the egg near the centre of the white 

 spot, by a delicate filament. A nucleus is no longer to be found ; once, 

 however, when the polar globules were both formed, the nucleus [female 

 pronucleus'?] could be distinctly seen immediately under the surface of 

 the egg, but there was no nucleolus ; the nucleus was homogeneous, and 

 looked like a vacuole. There is a nucleus-like corpuscle inside the polar 

 globules, which gives to them the appearance of small cells. 



Rabl gives an account (76, pp. 316, 317, Taf X. Figs. 4-6 C), 

 and apparently very accurate figures, of the formation of the second 

 polar globule in Unio, so far, at least, as can be seen on living eggs. 

 More than two polar globules were never observed, nor was the second 

 ever produced by a division of the first. The first is usually somewhat 

 larger than the second. He also figures at the vegetative pole a cone- 

 like elevation, ^ which has not entirely disappeared when the second 

 globule is forming. The egg is without a germinative yesicle. 



In a lengthy consideration of the significance of the polar globules 

 (pp. 331 - 338) Rabl combats the notion that their elimination is 

 comparable to an act of defecation ; for one would then be compelled, 

 he says, to assume quite different physiological processes for the first 



