THE ZOOLOGICAL STATION AT NAPLES. 283 



The following points were made out in the larger eggs before treatment 

 with reagents. 



The eggs are perfectly spherical and transparent. The nurse-cells, 

 if still present, occupy a small area at one pole. There is no follicle 

 around the egg, but an extremely fine membrane {1 zona) — which therefore, 

 as Bergmann points out for Onuphis, must be an independent product 

 of the egg itself — is present at the surface. In the cytoplasm just under 

 the membrane minute highly refracting droplets, probably of oil, can be 

 discerned. They are often in clusters and of various sizes. In the very 

 centre of the egg is the perfectly spherical germinal vesicle with a single 

 highly refractive germinal spot or nucleolus. Occasionally one or two 

 smaller refractive bodies (the ' neben-nucleoli ') may be seen within the 

 germinal vesicle. The position of the germinal spot is invariably 

 eccentric. Vacuoles varying in number and size could be distinguished 

 in the nucleolus, except in the case of the largest eggs. The nucleolus 

 of the full -sized eggs was notably smaller and at times contained a small 

 vacuole. The nucleokis, therefore, enlarges up to a certain point in the 

 growth of the oocyte and then diminishes. Its complete dissolution was 

 not observed. 



The space between the germinal vesicle and the egg membrane is 

 filled up with yolk spheres. These are nearly uniform in size, and almost 

 touch one another, leaving very little protoplasm between. The spheres 

 are not very highly refractive, and are therefore only vaguely discernible 

 in the fresh egg. 



In the germinal vesicle of the full- sized eggs one can distinguish, 

 besides the nucleolus, certain nebulous or flocculent masses. These are 

 the definitive chromosomes. To see them in the fresh egg requires a 

 certain intensity of light and careful focussing. 



By the addition, under the cover-glass, of an aqueous solution of 

 methyl green more facts were brought to light. As the green solution 

 reaches the eggs these swell up somewhat and burst their membranes. 

 Often the yolk is extruded in small drops through the substance of the 

 membrane, the external surface of which is accordingly studded with 

 drops. This observation seems to indicate that the egg membrane of 

 Tomopteris, like the zona radiata of vertebrates, is perforate. The 

 protoplasm flows out through a rupture in the capsule slowly, sometimes 

 in long strings like a syrup. The yolk spheres entangled in it generally 

 adapt themselves to the size of the aperture and pass out intact. On 

 coming in contact with the watery solution they break down and flow 

 together. The yolk spheres are, I think, evidently viscid drops of some 

 albuminous substance. Inside the egg the syrupy protoplasm in 

 which the spheres are imbedded appears quite homogeneous and trans- 

 lucent, but as it flows out into the watery methyl-green solution minute 

 granules (? microsomes) come into view in its interior. It is probable 

 that, as Wilson ' has observed in the case of certain Echinoderm and 

 Annelid eggs, the yolk of the Tomopteris egg forms a true emulsion in 

 Biitschli's sense. I cannot, however, definitely state the existence of 

 microsomes in the cytoplasm of uninjured eggs, i.e. before contact with 

 the methyl-green solution. Probably they are naturally present in this 

 transparent egg and only require a coloured solution, like methyl green, 

 to show them up. 



' Wilson, Journal of Morphology, vol. xv., Supplement, 1899. 



