DEVELOPMENT OF PBTROMYZON FLUVIATILIS. 175 



where the yolk granules in the more quickly dividing upper 

 pole are much smaller than those in the more inert lower pole. 

 An attempt has been made to show that those parts of the 

 unsegmented egg containing the smaller granules is destined 

 to form the epiblastic parts of the embryo (16).^ This view 

 seems to me to need confirmation. The small size of the yolk 

 granules in the epiblast might be due to the more rapid 

 division of these cells, causing a more rapid consumption of the 

 food-yolk. 



The unusually deep staining which the yolk granules assume 

 very materially increases the difficulty of observation. Espe- 

 cially in the earlier stages of development the cell limits and 

 nuclei were rendered obscure by the masses of deeply stained 

 yolk granules. 



As previous observers have stated, there are two polar bodies 

 extruded one after the other. After fertilization the egg con- 

 tracts, leaving a cavity between it and the egg membrane. 



The first furrow appears about the fourth hour ; it appears 

 first in the upper pole and spreads round the egg on each side. 

 Calberla states that the micropyle becomes at first oval, then 

 slit like, and finally passes over into the primary furrow. I 

 have not been able to observe this process in my eggs. He 

 further states that the first furrow divides the egg into two 

 unequal parts, a large epiblastic and a small hypoblastic ; the 

 smaller of these divides subsequently more rapidly than the 

 latter. Thus, according to him, the first furrow would cor- 

 respond with the first equatorial one in the Frog's ovum. 

 Scott, although he had no fresh material to work with, was 

 able to correct this, and, as the latter suggests, Calberla was 

 probably misled by cases of abnormal segmentation. Many 

 of the eggs which apparently had not been fertilized divided 

 by one, two, and sometimes three furrows, and when this 

 took place the furrows were nearly always abnormal in 

 position. 



The second furrow is vertical and at right angles to the 

 first, and also appears first in the upper pole. The third is 

 ' The figures in brackets refer to the list of papers at the end. 



