236 MARY BLOUNT. 



kugel." The other daughter nucleus passes into the marginal 

 segment, and so on until finally the part of the marginal segment 

 left over, changes over into a " Furchungskugel." And so, ac- 

 cording to Kolliker, the first appearing Furchungskugeln are 

 never completely cut off from the unsegmented Bildungsdotter 

 below, but nuclei, sisters to those in the first layer of cells, pass 

 down into the Bildungsdotter. Here nuclear division takes place, 

 and cells are organized around the upper daughter nuclei, thus 

 forming the second layer of cells in the center of the blastodisc, 

 while the lower daughter nuclei are left deeper in the " Bildungs- 

 dotter." And thus cleavage proceeds downward until finally the 

 last remaining nucleated portions of the " Bildungsdotter " change 

 over into "Furchungskugeln." 



In the pigeon's egg, on the contrary, I do not find any such 

 deepening of the center of the blastodisc. The change from one 

 layer to several layers of cells is by a process exactly like that of 

 the teleost egg. See Agassiz and Whitman (1) Fig. 2, and Wil- 

 son (8) Figs. 16, 17, 18 and 19. The blastodisc of the pigeon's 

 egg becomes stratified by horizontal cleavage planes arising above 

 the first horizontal cleavage ; i. e., above the level of the plane 

 which limits the cell b below (Fig. 3). Nuclei are never found in 

 the central part below the level of the horizontal cleavage under the 

 cell b. 



Extending deep into the white yolk is a cone of slightly gran- 

 ular protoplasm. It varies in extent in different stages as will be 

 seen by comparing Figs. 3, 5, 6 and 9. A more central section 

 than Fig. 5 shows this cone extending deeper. In some stages 

 it is better described as being funnel-shaped, with the slender 

 tube of the funnel going deep into the yolk, and the mouth open- 

 ing on the the lower side of the blastodisc. In some of the sec- 

 tions of the egg represented in Fig. 9 it is found at twice the 

 depth of the figure. If the sections were cut exactly perpendicu- 

 lar to the surface, the funnel would appear continuous from 

 its broad end at the blastodisc to the deepest limit included in 

 the section. A similar structure has been figured by Eycle- 

 shymer (2) for the egg of Lepidosteus osseus, Figs. 32, 34 and 

 others. He calls it, " the peculiar conical prolongation of the 

 periblast." 



