46 DEVELOPMENT OF THE FROG'S EGG [Cn. IV 



than a', b" is less than b', but A' is less than A" and B' is 

 less than B", then the drops assume the form shown in 

 Fig. 17, D. 



If we divide four equal drops (Fig. 16, B) so that a' is less 

 than a", b" is less than 6', A" is less than A', and B' is less than 

 B", the resulting eight drops arrange themselves as shown in 

 Fig. 17, E. In this system it is instructive to note how far 

 the first division-plane is drawn out of its straight course as 

 a result of the shifting of the drops on one another. It is not 

 unusual for one of the drops to glide into the centre of the 

 system, as shown in Fig. 17, F. This produces a more stable 

 arrangement than when a large central cavity is present. 



Most of these systems are found also in the segmenting frog's 

 egg, as can be seen by comparing these figures of the oil-drops 

 with the figures of the segmenting frog's egg (Fig. 13) by Max 

 Schultze made in 1863. Rauber has also given figures showing 

 arrangements of the upper eight blastomeres (Fig. 14), like 

 the systems of oil-drops shown in Fig. 17, D and E. 



A careful comparison between the systems of oil-drops and 

 the arrangement of the blastomeres of the frog's egg shows, 

 as Roux points out, that while in many cases the agreement 

 is perfect, yet occasionally the blastomeres assume an arrange- 

 ment that oil-drops of the same size would not assume. For 

 instance, Roux figures an arrangement of the blastomeres like 

 that of Fig. 17, C, but here the blastomere corresponding to a' 

 is less than a" and A' is less than A". In this egg the smaller 

 blastomeres meet in the centre, but this never occurs in the 

 system of oil-drops. 



Roux removed a part of a blastomere so that it became sud- 

 denly smaller. A new arrangement ought now to have taken 

 place among the blastomeres if they conformed entirely to the 

 laws regulating the oil-drops. In one case where four blasto- 

 meres were present, the blastomere that had been reduced in 

 size did move out more toward the periphery of the system, 

 and the two neighboring blastomeres pushed in more toward 

 the centre to form a cross-line. In other experiments, how- 

 ever, the blastomeres did not rearrange themselves in con- 

 formity with the systems of oil-drops. For instance, in one 

 experiment in which material was drawn out of one of the first 



