98 DEVELOPMENT OF THE FROG'S EGG- [Ch. X 



tigure (Fig. 31, B). The fourth furrows are also vertical {i.e. 

 at right angles to the plates) and generally run parallel to the 

 second planes of cleavage, as seen in the figure (Fig. 31, A, B). 

 There is no segmentation-cavity as yet present in these com- 

 pressed eggs. 



It is possible to keep these eggs in position until the blasto- 

 pore appears, and then to follow its movements up to a time 

 when the medullary folds form. The blastopore appears on 

 the under side, i.e. on the white hemisphere near the edge of the 

 egfi. It closes at the opposite edge of the lower surface. The 

 medullary folds also appear on the lower surface of the Qgg-, and 

 remain there until the embryo begins to lengthen. The belly 

 is therefore turned upward. 



2) Efifjs compressed latercdly, i.e. at right angles to the primary/ 

 axis, zvith the black pole kept upward (Fig. 30, B). The eggs 

 were placed between glass plates so that, when the plates were 

 turned vertically, the axis of the eggs also stood vertical, and 

 the compression was from the sides. The hrst furrow is ver- 

 tical and at right angles to the two glass plates (Fig. 31, C, D). 

 The furrow passes through the middle of the egg., dividing it 

 into two equal parts. Deviations from this mode of division 

 often occur. The first division sometimes passes obliquely, i.e. 

 to one side from above downward, but keeps always at right 

 angles to the glass plates. 



The second cleavage comes in also at right angles to the 

 plates, and at right angles to the first furrow, and therefore in 

 a horizontal position. It always lies nearer the upper (i.e. the 

 black) side of the agg, as shown in the figure (Fig. 31, C, D). 

 Two upper small cells and two lower large cells are formed. 

 The second furrows have come in where, normally, the third 

 furrows lie. 



The furrows of the third order appear first in the upper 

 smaller cells. They are at right angles to the glass plates, and 

 parallel to the first furrow, near to which thej^ often lie (Fig. 

 31, C, D). Occasionall}^ a furrow of the third order may lie 

 parallel to the second, and not to the first furrow ; it may even 

 run along the edge of the compressed Qgg, and is then parallel 

 to the compressing plates. In the loAver cells the furrows of 

 the third order also come in vertically and at right angles to 



