408 BtTLLETIN 82, UNITED STATES NATIONAL, MUSEUM. 



deviation from this form which is seen in fixed material is an artifact, due to the 

 action of the fixing reagent at a time when the osmotic intensity has been consider- 

 ably lessened by changes in the cytoplasm. The large size of the germinal vesicle 

 is to be explained as due to the same cause as that which brought about the fine 

 subdivision of the chromatin, namely, the limited growth allowed to this material 

 in the germ cell. For it is obvious that there are two ways of enabling a small 

 amount of chromatin to perform readily the utmost possible amount of work. 

 One of these is the great increase in surface area which is brought about by extreme 

 subdivision, wherebj' the facilities for reaction with the caryolympli are greatly 

 augmented ; the other is the increase, both in quantity and in nutritive value, of the 

 caryohnnpli itself, whicli is brought about by an increase in the surface area of the 

 nuclear membrane tlirough which diffusion takes place, the size of the germinal 

 vesicle being determined when equilibrium is established between the rate of diffu- 

 sion of nutritive substances through the nuclear membrane on the one hand, and the 

 requirements of the chromatin on the other. 



RIPENING OF THE EGGS. 



Hamann has studied in detail the ripening of the eggs in TIeliometra glacialis. 



As soon as the eggs have reached their full size they move into the central 

 cavity of the swollen portion of the genital tube, and often fill it entirely up. They 

 have a diameter of 0.3 mm. 



These eggs have lost their germinal vesicles, having in place of them a large 

 spherical nucleus of homogeneous appearance 0.01 mm. in diameter. The vitelline 

 membrane stands out prominently. On the periphery there lie two polar bodies, 

 of which the second is oval in shape and shows many chromatin granules; it lies on 

 the periphery with its longer axis at right angles to the cell wall. 



EXTRUSION OF THE EGOS. 



Pig. 1175, pi. 28. 



There are no female genital openings in the crinoids. As the eggs approach 

 maturity a small hemispherical swelling appears on the ventral side of the gravid 

 pinnule, which is noticeable on account of its whitish color. This is due to the 

 bulging outward of the wall of the genital tube containing the eggs, which pushes 

 against the previously somewhat sunken median portion of tlie ventral wall of the 

 pinnule. As the pressure increases the wall of the genital tube is forced more and 

 more outward into the pinnule wall, which finally opens and, the wall of the genital 

 tube tearing, the eggs are liberated through relatively small slits. 



During this process the egg case is formed about the egg cell. 



After the extrusion of the eggs the rent in tlie pinnule wall is closed, the wall 

 of the genital tube is withdrawn, and a rapid healing and regeneration takes place 

 in the pinnule wall, which is accompanied by accelerated cell proliferation. 



In Thaumatometra nutrix the eggs, which are only about 0.15 mm. in diameter, 

 when ripe pass through the wall of the pinnule into the marsupium, where fer- 

 tilization takes place. 



