496 CHARLES H. SWIFT 



3 micra. These red-like mitochondria do not as a rule have a 

 homogenous stiaicture, but are beaded or granular rods; that is, 

 beads or granules closely approximated and arranged in a row or 

 rod. At the same time in all stages discrete granules may be 

 present, but they are never seen to the exclusion of the rod form 

 unless the fixation has been faulty. 



This is at variance with the work of Rubaschkin ('10) and 

 Tschaschin ('10), who found, the former in mammals and the lat- 

 ter in birds, that the primordial germ-cells are characterized by 

 granular mitochondria, while those of the somatic cells are 

 rodlike in character. 



Size of the 'prijnordial germ-cell, nucleus and attraction-sphere. 

 The size of the germ-cell varies little, if any, from its origin, to its 

 entrance into the indifferent gonad. The average diameter of 

 nearly every one is found between the figures 14 and 18 micra. 

 The majority approach the larger figures. 



The nucleus undergoes a little variation in size. That of the 

 germ-cells of the primitive streak stage to 9 somites has a diameter 

 of 8 to 10 micra. In germ-cells of embryos possessing more than 

 12 somites the nuclear diameter varies between 10 to 12 micra. 



The attraction-sphere has a constant long diameter of 3 to 4 

 micra. This relatively constant size of the primordial germ-cell, 

 and of its organs, the nucleus and attraction-sphere, together 

 with the retention of yolk, after its disappearance from the so- 

 matic cells, may be taken as an evidence that the germ-cells do 

 not divide during their migration history, 



2. Migration of the germ-cells 



In investigating the history of the primordial germ-cells of 

 the chick it was found to be expedient to commence the study on 

 older embryos. The advantages of this method of study are 

 obvious; the older embryos are larger and therefore much easier 

 to handle than the younger ones, and in addition the germ-cells 

 have been observed down to embryos of about 22 somites and 

 described and studied by several able investigators — notably 

 Hoffman ('93), Nussbaum ('01), Rubaschkin ('07), Tschaschin 

 ('10) and von Berenberg-Gossler ('12). Therefore, in begin- 



