GROWTH AND DIFFERENTIATION OF THE GERM-CELLS I 1 9 



(Jordan, '93) and myriapods (Balbiani, '93), where the egg contains a 

 number of fairly well defined yolk-nuclei. In Lumbricus the "yolk- 

 nucleus " first appears as a single irregular deeply staining body 

 closely applied to the nucleus and afterwards breaks up into numer- 

 ous smaller bodies (Calkins, '95). 



The most diverse accounts have been given of the structure and 

 origin of these problematical bodies. This is in part owing to the 

 fact, recently pointed out by Mertens, that two entirely different 

 structures have been confounded under the one term. One of these 

 is the attraction-sphere of the young egg with its centrosome. Such 

 a "yolk-nucleus" has been described by Balbiani in the eggs of the 

 myriapod Geophilus (Fig. 59, B}. The other is a body, variously 

 described as arising from the nucleus or in the cytoplasm, which is 

 not improbably concerned in some manner with the constructive 

 metabolism involved in the growth of the egg-cytoplasm and perhaps 

 indirectly concerned with the formation of deutoplasm. It seems 

 clear that the latter form alone should receive the name of yolk- 

 nucleus, if indeed the term is worth retaining. 



Mertens ('93) has recently described the ova of a number of birds 

 and mammals (including man) as containing a very distinct attrac- 

 tion-sphere containing one or more intensely staining centrosomes 

 (Fig. 60). This has, however, nothing to do with the true yolk- 

 nucleus which may sometimes be seen in the same egg, lying beside 

 the attraction-sphere (Fig. 60, D). The latter sooner or later fades 

 away and disappears. The yolk-nucleus, on the other hand, may long 

 persist. This observation probably explains the strange result reached 

 by Balbiani in the case of myriapods (Geophilus}, where the "yolk- 

 nucleus " is described as arising by a budding of the nucleus, yet is 

 identified with an attraction-sphere ! The "yolk-nucleus " of Balbiani 

 has here the typical appearance of an attraction-sphere, surrounded 

 by rays and containing two or several centrosomes or centrioles. 

 Besides this, however, the egg contains several other bodies which 

 are described as arising by budding off from the nucleus and per- 

 haps represent the true yolk-nuclei (Fig. 59, B). 



The origin of the yolk-nucleus proper appears to differ in different 

 cases. Jordan's observations on the newt seem to leave no doubt 

 that the bodies described as yolk-nuclei in this animal arise in situ in 

 the cytoplasm ; and a similar origin of the yolk-nucleus has been 

 described by a number of earlier observers. On the other hand, a 

 number of observers have asserted its origin from the nucleus, either 

 by a process of nuclear budding, by a casting out of the nucleolus of 

 separate chrornatin-rods, or of portions of the chromatic reticulum. 

 That such a casting-out of nuclear substance occurs during the ova- 

 rian history of some eggs appears to be well established ; but it is 



