48 



the egg is still uncovered with cells. The nuclei here lie embedded 

 in the peripheral protoplasm, which has become much thinner. There 

 are still many nucleated stellate cells scattered throughout the yolk. 

 This stage, which represents conditions at the beginning of the sec- 

 ond day, is illustrated by Plate II, Figure 8. 



During the second day (PI. Ill, Fig. 9) cells are formed over the 

 entire surface of the posterior half of the egg in the same way that 

 they were formed over the ventral surface of the anterior half 

 the first day. These cells differ from those at the anterior end in 

 being much larger and broader. The protoplasm is mostly at the 

 distal end of the cell, while the basal part contains an immense num- 

 ber of yolk granules. At this stage those cells at the posterior end 

 contain also' a very large number of the bacteria mentioned above. 

 The nuclei in these cells lie in the distal ends, as do those of the an- 

 terior end, but they are not nearly so easily made out. On the ventral 

 surface the cells of the posterior half meet those of the anterior half 

 about half-way between the two poles to form a continuous layer. 

 The transition from one type of cell to that of the other is not a 

 gradual one but is rather abrupt. On the dorsal surface the layer 

 of cells from the posterior end has grown forward about the same 

 distance as on the ventral side, but the layer of cells from the an- 

 terior end has not grown backward to meet it, so there is still a 

 small area on the dorsal surface just back of the anterior end that 

 is as yet not covered with cells. The protoplasmic layer has changed 

 here until there is nothing but a thin membrane separating the yolk 

 from the vitelline membrane (Fig. 10). 



The cells that were described above as being formed the first 

 day on the ventral surface of the anterior half have changed de- 

 cidedly in appearance. They have become longer and more columnar 

 and taper somewhat at the base. This layer of cells forms the be- 

 ginning of the germ band. At their proximal ends they merge into 

 the protoplasm which has formed a layer lying between the yolk and 

 the cells. In the posterior half this layer almost disappears in the 

 protoplasmic network; but in the anterior half it is much heavier 

 and really forms a syncitium, since it contains quite a number of 

 nuclei. This syncitium seems to act as a kind of "feeder" to the 

 layer of cells, since it is the seat of rapid nucleus formation, nuclei 

 appearing here in various stages of mitosis, and since the nuclei be- 

 ing formed here appear to migrate outward, drawing with them a 

 part of the syncitial protoplasm, thus forming new cells. These mi- 

 grating nuclei can be found at the distal ends of the cells just form- 

 ing in this way, anywhere between the protoplasmic layer and the 



