MONOGRAPH OF THE EXISTING CRINOIDS. 389 



In the young oocyte the nucleolar spherules in the cytoplasm increase in 

 number with every discharge from the nucleolus, but otherwise remain unchanged. 

 The cytoplasm also at this period shows no other change than a considerable 

 increase in bulk and a veiy slight increase in basophile staining reaction. Sooner 

 or later, however, and alawys by the time the nucleolus measures 8 [a, in eggs, 

 therefore, of from 20;;. to 30 ji in diameter, a portion of the hitherto uniformly 

 staining cytoplasm commences to retain the basic stain. This more deeply stain- 

 ing region of the cytoplasm is at first extremely irregular and diffuse in character 

 and nearly always stands in relation to some, at least, of the groups of nucleolar 

 spherules, and the appearance thus presented strongly suggests that a diffusion 

 is taking place of the deeply staining substance of the nucleolar spherules onto the 

 surrounding cytoplasm. During this diffusion the sharply marked and deeply 

 stained nucleolar spherules become more and more scarce, and at the same time the 

 more deeply staining region of the cytoplasm gradually loses its irregular and 

 scattered condition, becoming instead compact and regular, and assumes the form 

 of a faintly staining but clearly defined concavo-convex, lens-shaped body, closely 

 applied to, though never in actual contact with, the surface of the spherical nucleus 

 and measuring '25 u. to 30 u. in diameter and from Sja to 10 (j. in tichkness. This 

 is the definitive yolk nucleus. 



That the more deeply staining area of the cytoplasm, and consequently also 

 the definitive yolk nucleus, does not arise by a simple aggregation of the nucleolar 

 spherules is shown by the difference in staining reaction of the yolk nucleus and 

 of the nucleolar spherules. Professor Chubb gives a figure representing an egg 

 fixed in sublimate showing two groups of nucleolar spherules which have only 

 just come into relation with the stainable area; their deeply basophile character 

 and sharp outline offer a striking contrast to the somewhat diffuse and less deeply 

 stained spherules of the more deeply staining cytoplasmic area. This contrast 

 was still more striking with material fixed with other reagents; thus with Her- 

 mann, or with acetic bichromate, the nucleolar spherules are, as with the sub- 

 limate, deeply stained and clearly defined, while the texture of the yolk-nuclear 

 area, like that of the rest of the cytoplasm with these reagents, is finely granulo- 

 reticular in character, only differing from the surrounding cytoplasm in its greater 

 affinity for the basic stain. 



Although the accumulated nucleolar spherules in the cytoplasm greatly 

 diminish in number during the formation of the yolk nucleus, and may even be 

 entirely absent on the completion of this process, they nevertheless are still 

 occasionally to be seen in eggs in which a definitive yolk nucleus is present. The 

 rapid increase in staining capacity shown by the yolk nucleus soon after its 

 appearance, and which is described below, is in all probability to be explained 

 by this incorporation of additional nucleolar spherules. At later stages, as the 

 nucleolar discharges become more and more infrequent, spherules in the cytoplasm 

 are more rarely met with, and are never found in contact with the yolk nucleus 

 after this has lost its intensely basophile character, although even in the adult 

 egg groups of partially dissolved nucleolar spherules are occasionally to be seen 

 embedded in the volks. 



