406 CHARLES H. SWIFT 



cent appears denser and stains darker than that of the rest of 

 the cell (figs. 4, 5 and 6). 



This mitochondrial crescent appears in cells which are the 

 result of primordial germ-cell division, in other words, in the 

 oogonia and spermatogonia. It appears in a few cells in the 

 female at 8 days and in the male at 13 days and this marks the 

 ending of the primordial germ-cell line and the beginning of the 

 oogonial and spermatogonia! generations. By 17 days in the 

 male (fig. 4) eveiy cell of primordial germ-cell lineage possesses 

 it, thus indicating that all are spermatogonia. When the sper- 

 matogonia place themselves against the basement membrane, 

 when the cavity begins to appear in the cords, they do so in 

 such a manner that the mitochondrial crescent is against the 

 basement membrane (figs. 5 and 6). 



This mitochondrial crescent, whose appearance and formation 

 I have just described, is the same structure which D'HoUander 

 ('04) described in the oocytes of the female chick. He, how- 

 ever, called the central sphere and its surrounding clear area 

 the yolk nucleus of Balbiani and the mitochondrial portion the 

 'couche vitellogene' or 'couche palleale.' His methods did not 

 demonstrate the mitochondria, but in the region of the mito- 

 chondrial crescent which they occupy his text described, and his 

 figures showed a denser zone of cytoplasm. This denser zone 

 may be dissolved mitochondria or it may be more condensed 

 cytoplasm, which seems also to be present in mv preparations 

 (fig- 4). 



There is, of course, a great deal of dispute as to the nature 

 of the yolk nucleus — its cytological structure and its role in the 

 cell. 



Mertens ('94), in his study of the yolk nucleus of Balbiani in 

 the oocytes of birds and mammals, came to the conclusion that 

 two very different entities are described under the one name, 

 first certain elements found in the cytoplasm which originate 

 in the nucleus and, secondly, the attraction-sphere, which van 

 Bambeke called the 'couche palleale' and Van der Stricht the 

 'couche vitellogene.' At the present moment I am not pre- 

 pared to say am^thing positively as to the origin of the denser 



