GERM-CELL HISTORY IN THE BROOK LAMPREY 21 



that the extruded cells are germ cells which have failed to 

 reach the germ-gland anlage. 



0. Larva 20 mm. long. This larva is about four months old. 

 The germ fold has now formed (fig. 27) and extends along the dor- 

 sal wall of the coelom as a low longitudinal ridge. Some of the 

 germ cells have migrated into the fold, but others are still in the 

 mesenchyme above it. Their position is such that they are prac- 

 tically surrounded by blood-vessels — the posterior cardinal veins 

 laterally and the dorsal aorta above. Besides these vessels, a 

 large number of smaller vessels permeate the tissues around the 

 germ cells. The germ cells occur in groups and in some places 

 long distances intervene between them so that the various groups 

 do not form a continuous band. The germ cells are more 

 numerou-^ than in the preceding stage, and in ome cases two are 

 found which apparently are surrounded by a single follicular 

 membrane — an indication of recent division. In most of the 

 cells at this stage, a distinct attraction sphere occurs. It con- 

 sists of a mass of closely set granules and hes against one side of 

 the nucleus. It may cover as much as one-third of the circum- 

 ference of the nucleus in each section through the middle of the 

 cell (fig. 28). No distinct centrosome could be found. Besides 

 the attraction sphere, there is in the cytoplasm a spindle-shaped 

 body, the 'vitelline body' of King ('08), which is much smaller 

 and is made up of coarser granules than the attraction sphere. In 

 longitudinal section it appears oval, in cross-section, round. 

 Judging from sections through various planes, it is shaped like a 

 spindle which tapers abruptly at both ends. It may occur 

 almost anywhere in the cytoplasm, sometimes near the nucleus 

 but sometimes close under the cell membrane. The origin of 

 this body could not be ascertained. The presence of a centro- 

 sphere indicates the beginning of mitotic activity. The cells 

 may now be considered as having passed out of the primary 

 period of rest and entered the period of secondary division. 

 There is yet no indication of sexual differentiation and, in the 

 absence of any characters which distinguish the secondary sper- 

 matogonia from the secondary oogonia, the germ cells may be 

 regarded as still indifferent as to sex (table 2). 



