158 THE GERM-CELLS 



even after the yolk-nuclcus has ijuite separated from the nuclear 

 membrane. Later, however, as the yolk-nucleus breaks up, it changes 

 its stainini;- power, and stains red like the cytoplasm. The later 

 observations of Miss Foot ('96) give ground to doubt the conclusion 

 that the yolk-nuclcus is here actually metamorphosed chromatin, 

 for by the combined action of Hthium carmine and Lyons blue its 

 substance is sharply differentiated from the chromatin. Still later 

 studies by Crampton ('99) on Molgula demonstrate that in this case 

 the yolk-nuclcus is not directly derived from chromatin, but they 

 nevertheless indicate clearly the formation of the yolk-nucleus by or 

 under the immediate influence of the nucleus — a conclusion also 

 reached on less satisfactory evidence by Hubbard, Van Bambeke, 

 Woltereck, and Nemec. The general morphological history of the 

 yolk-nucleus is here closely similar to that of Luuibriciis (Fig. 80), 

 except that no direct continuity between it and the nuclear substance 

 was observed. Stained with methyl-green-fuchsin the yolk-nucleus 

 and major part of the nuclear substance stain red, while the scattered 

 nuclear chromatin-granules and the cytoplasm stain green. Millon's 

 test, combined with digestion-experiments and the foregoing staining- 

 reactions, proves that the yolk-nucleus and the red staining nuclear 

 substance consist of albuminous substance and differ widely from 

 the general cytoplasm, which probably consists largely of nucleo- 

 albumins {cf. p. 331). These reactions give strong ground for the 

 conclusion that the substance of the yolk-nucleus, which progressively 

 accumulates just outside the egg-nucleus, is formed through the direct 

 activity of the latter, perhaps arising within the nucleus and passing 

 out into the cytoplasm. It is possible, further, that even the scattered 

 " yolk-nuclei " that seem to be of purely cytoplasmic origin may arise 

 in a similar manner, either, as Crampton suggests, through the early 

 formation and breaking up of a single yolk-nucleus, or in some less 

 obvious way. 



Interesting questions are suggested by those "yolk-nuclei," such 

 as occur in Gcophiliis and Lij)iulns, that so closely simulate an 

 attraction-sphere. Munson's observations show that this body 

 ("vitelline body") first appears in the very young ova as a crescent 

 applied to the nucleus precisely as in JMolgula or Liunbriciis, hut 

 containing one or more central granules (Fig. 81). In later stages 

 it becomes spherical, moves away from the nucleus, and assumes the 

 form of a typical radial attraction-sphere with concentric microsome- 

 circles and astral rays. It is hardly possible to doubt that this body 

 in Linmlus is of the same general nature as the yolk-nucleus of 

 Liivtbriciis, Molgula, Cypris, Cymatogaster, or P/iolciis ; and if it be 

 a true attraction-sphere in the one case we must probably so regard 

 it in all. This identification is, however, by no means complete ; 



