VIII] THE SEGMENTATION OF THE EGG 379 



possible, the arrangement of boundaries which probably actually 

 existed, and gave rise to the appearance which the observer drew. 

 These drawings may be compared with the various diagrams of 

 Fig. 158, in which some seven out of the possible thirteen arrange- 

 ments of five intermediate partitions (for a system of eight cells) 

 have been already set forth. 



It will be seen that M. Robert-Tornow's figure of the segmenting 

 egg of Trochus (Fig. 160) clearly shews the cells grouped after the 

 fashion of Fig. 158, a. In like manner, Mr Conklin's figure of the 

 ascidian egg {Cynthia) shews equally clearly the arrangement g. 



A sea-urchin egg, segmenting under pressure, as figured by 

 Driesch, scarcely requires any modification of the drawing to 

 appear as a diagram of the type d. Turning for a moment to a 

 botanical illustration, we have a figure of Pringsheim's shewing an 

 eight-celled stage in the apex of the young cone of Salvinia ; it 

 is in all probability referable, as in my modified diagram, to type 

 c. Beside it is figured a very different object, a segmenting egg 

 of the Ascidian Pyrosoma, after Korotneff ; it may be that this 

 also is to be referred to type c, but I think it is more easily referable 

 to type b. For there is a difference between this diagram and 

 that of Salvinia, in that here apparently, of the pairs of lateral 

 cells, the upper and the lower cell are alternately the larger, while 

 in the diagram of Salvinia the lower lateral cells both appear much 

 larger than the upper ones ; and this difference tallies with the 

 appearance produced if we fill in the eight cells according to the 

 type 6 or the type c. In the segmenting cuttlefish egg, there 

 is again a slight dubiety as to which type it should be referred to, 

 but it is in all probability referable, like Driesch's Echinus egg, 

 to d. Lastly, I have copied from Roux a curious figure of the 

 egg of Rana esculenta, viewed from the animal pole, which appears 

 to me referable, in all probabiHty, to type g. Of type/, in which 

 the five partitions form a figure with four re-entrant angles, that 

 is to say a figure representing the five sides of a hexagon, I have 

 found no examples among segmenting eggs, and that arrange- 

 ment in all probability is a very unstable one. 



It is obvious enough, without more ado, that these phenomena 

 are in the strictest and completest way commoii to both plants 



