156 THE GERM-CELLS 



mass which appears at a very early period and persists throughout 

 the later ovarian history. In other forms there are several so-called 

 " yolk-nuclei," sometimes of fairly definite form as described in the 

 Amphibia by Jordan ('93) and in some of the myriapods by Balbiani 

 ('93). In some forms the numerous "yolk-nuclei" are irregular, ill- 

 defined granular masses scattered through the cytoplasm, as described 

 by Stuhlman ('86) in the eggs of insects. In still others the "yolk- 

 nucleus" or "vitelline body" closely simulates an attraction-sphere, 

 being surrounded by distinct astral radiations and enclosing one or 

 more central granules like centrosomes (Geophilus, Balbiani, '93, and 

 Limulus, Munson, '98). Balbiani is thus led to regard the yolk- 

 nucleus in general as being a metamorphosed attraction-sphere. 

 Miss Foot ('96) has brought forward evidence to show that the polar 

 rings, observed in the eggs of certain leeches and earthworms, are 

 also to be regarded as "yolk-nuclei" (Fig. 102). Henneguy ('93, 

 '96) finally compares the yolk-nucleus to the macronucleus of the 

 Infusoria (!). 



In the present state of the subject it is quite impossible to reconcile 

 the discordant accounts that have been given regarding the structure, 

 origin, and fate of the " yolk-nuclei " , and from the facts thus far 

 determined we can only conclude that the various forms of " yolk- 

 nuclei" have little more in common than the name. It is, in the 

 first place, doubtful whether the " yolk-nuclei " simulating an attrac- 

 tion-sphere have anything in common with the other forms ; and 

 Mertens ('93), Munson ('98), have shown that the young ovarian ova 

 of various birds and mammals (including man) and of Limttltis 

 contain one or more "yolk-nuclei" in addition to the "attraction- 

 sphere " ("vitelline body" of Munson). In the second place there 

 seem to be two well-defined modes of origin of the yolk-nucleus. In 

 one type, illustrated by Jordan's observations on the newt ('93), the 

 " yolk-nuclei " arise separately in situ in the cytoplasm without direct 

 relation to the nucleus. The same is true of the small peripheral 

 " yolk-nuclei " of Limulus (Munson). In a second and more frequent 

 type the "yolk-nucleus" first appears very near to or in contact with 

 the nucleus, suggesting that the latter is directly concerned in its 

 formation. The latter is the case, for example, in the eggs of Cyma- 

 togaster (Hubbard, '94) Syngnathus (Henneguy, '96), the earthworm 

 (Calkins, '95, Foot, '96), Polyzonium and other myriapods (Nemec, 

 '97, Van Bambeke, '98), Limulus (Munson, '98), Cypris (Woltereck, 

 '98), and Molgula (Crampton, '99). I n nearly all of these forms the 

 yolk-nucleus first appears in the form of a cap closely applied to one 

 side of the nucleus (Figs. 80, 81), sometimes so closely united to the 

 latter that it is difficult to trace a boundary between them. At a 

 later period the yolk-nucleus moves away from the nucleus and in 



