82 EXTERNAL FACTORS III. i 



to answer by experiments, performed, however, not on the eggs 

 of the Frog but on those of a Toad, Bombinator igneus. 



The close adherence of the unlaid egg to the glutinous jelly, 

 which in its turn could be easily fixed to some object, provided 

 a simple method of keeping the egg in any desired position. 

 The eggs were removed from the uterus, attached with the axis 

 at various angles to the vertical to watch glasses and fertilized 

 with just enough sperm-water to allow of development, but not 

 enough to permit of the formation of the perivitelline space and 

 rotation of the egg into the normal position with the axis 

 vertical. In these forcibly inverted eggs it was found that the 

 furrows of segmentation bore the same relation to the vertical 

 as in the normal egg ; that is to say, the first was vertical, the 

 second vertical and at right angles to the first, the third hori- 

 zontal and nearer the upper pole, whatever the inclination of the 

 egg-axis to the vertical, except in the extreme case where the 

 white pole was exactly uppermost (180 of inversion), when 

 segmentation did not occur at all. There was, however, no 

 definite relation between the plane of the first (and therefore of 

 subsequent) furrows and the original axis of the egg ; the angle 

 between this axis and the plane of the first furrow, as also that 

 between the first furrow and the plane including the original 

 and the actual vertical axes of the egg, might, it was found, have 

 any value. 



Except in a few cases, and where the white area was nearly 

 exactly uppermost, these eggs gave rise to normal embryos. 

 The upper smaller cells divided more rapidly than the lower ones, 

 whether pigmented or unpigmented, and the blastula stage was 

 reached; the dorsal lip of the blastopore appeared on one side 

 a little below the (actual) equator, and the lower surface was 

 covered over by the blastoporic fold in the ordinary way. Only 

 in the failure of the whole egg to rotate after the closure of the 

 blastopore (owing to the close adherence of the jelly) and in 

 the irregular pigmentation (according to the original degree of 

 inclination) did these embryos differ from the normal. One 

 other point is worthy of notice. In the majority of cases the 

 dorsal lip of the blastopore, marking the sagittal plane, appeared 

 on the unpigmented side and lay in the plane including the 



