114 Incubation of the Top-Minnow (Gambusia). [February, 



time, may have served to develop specific characters, and that it is 

 undescribed. We leave the determination of the species to the 

 systematic ichthyologists. 1 



Thus far our account has dealt only with the structure of the 

 adults and the peculiar contrivances by means of which repro- 

 duction is effected ; we will now take up the discussion of the 

 egg and the embryo. 



The globular vitellus measures about a line in diameter includ- 

 ing the embryonic or germinal portion. The germinal proto- 

 plasm probably occupies a peripheral position covering the nutri- 

 tive or vitelline portion of the egg as a continuous envelope with 

 strands of germinal matter running from it through and among 

 the corpuscles of the vitellus. This peripheral germinal layer, 

 when the egg is ready to be fertilized, migrates toward one pole 

 and assumes a biscuit shape. This is essentially the history of 

 the formation of the germinal disk of the Teleostean egg as 

 worked out independently by Coste, Kupffer and the writer. 

 Little of a trustworthy character is known of the history of the 

 gerrninative vesicle and spot, which bear the same relation to the 

 egg as the nucleus and nucleolus do to the substance of the cell 

 of the ordinary type. When cleavage of the germinal disk has 

 begun, it is the first positive evidence that impregnation has been 

 successful. The disk then begins to spread over the vitellus or 

 yelk and soon acquires the form of a watch glass, with its concave 

 side lying next to the surface of the yelk. Coincident with the 

 lateral expansion of the germinal disk, a thickening appears at 

 one point in its margin which is the first sign of the appearance 

 of the embryo fish. With its still further expansion, the em- 

 bryo is developed more from the margin of the disk toward its 

 center; in this way it happens that the axis of the embryo lies in 

 one of the radii of the disk ; its head toward the center, its tail at 

 the margin. 



But before the embryo is fairly formed, a space appears under 

 the disk, limited by the thickened rim of the latter, and the em- 

 bryo at one side. This space, the segmentation cavity, 2 is filled 



