THE STAEFISH SOLASTER ENDECA. 7 



invaded by the yolk, until in the end there remains under the egg-membrane only 

 the thinnest possible yolk-free layer. 



It should be noted that while the gross aggregation of yolk takes place from the 

 nucleus outwards, as above described, stray granules may be observed in all stages at 

 lesser depths, or even close under the egg-membrane. Similar granules along with 

 many coarser ones occur within the follicle-cells, and it seems not improbable that 

 during growth the former are transferred through the vitelline membrane to the ovum, 

 where they congregate round the germinal vesicle, the stray granules above mentioned 

 being on their passage inwards from the periphery. A remarkable instance of trans- 

 ference of the granules is given on p. 45. 



The yolk-granules on their first appearance are small, rounded, and uniform in size. 

 As growth proceeds they become slightly larger everywhere, the increase in the full- 

 sized egg being least at the very surface close under the vitelline membrane. 



In the middle and later stages of growth, bodies resembling the so-called yolk- 

 nuclei are very evident. Their substance is finely punctate. They stain well in 

 heemalum, but with a characteristic reddish tinge. They are by no means regular 

 in number, size, and position, though the largest are always found quite close to 

 the germinal vesicle. They have no definite boundary, but merge at their periphery 

 into the protoplasmic reticulum in which the yolk-granules lie. They disappear 

 when the egg has reached full size and maturity. It seems probable that they are 

 formed by the activity of the nucleus (germinal vesicle), and that migrating outwards 

 from it they become broken up through contributing to the general protoplasmic 

 meshwork of the cell. Chubb (2, p. 384) describes the yolk-nucleus in Antedon as 

 formed by the throwing-out of small globules from the nucleolus. 



A vitelline membrane is recognizable from a very early stage. In larger ova it 

 remains thin, but is moderately tough, though not comparable in this respect with the 

 membranes of many other ova of similar size. It seemed to be perforated by numerous 

 fine pores, and may accordingly be termed a " zona radiata." 



Apart from distortion and facetting due to pressure, the shape of the ova within the 

 egg- tubes is already that of a spheroid with marked flattening at two opposite poles, 

 one of which is on the side of the ovum next the gonad-wall. This, the future upper 

 pole of the egg is bright orange in colour. The lower or yolk pole looks towards the 

 lumen of the egg-tube and is distinctly paler than the first. The transition between 

 the two colours is gradual, and occurs at the equator or widest part of the egg. The 

 diameter in the equatorial plane averages 1 mm., while the vertical diameter is 

 approximately '8 mm. 



Follicle-Cells. — The follicle-cells give a covering to the eggs, except where the 

 latter are attached to the gonad-wall, and also line such parts of the wall as have no 

 ova resting against them. Here, in places, they are folded inwards in the form of 



