8o CONK LIN. [Vol. XIII. 



the enteroblasts rotate into this position in advance of the ecto- 

 blastic cap. In such eggs as the one shown in Fig. 34 it is seen 

 that the entoblastic derivatives of the cell 4d have rotated farther 

 in an anti-clockwise direction than the overlying mesoblast and 

 ectoblast. Although the cell 4d was formed at a much earlier 

 period than the corresponding cells 4a, 4b, and 4c, it does not 

 rotate until the latter cells are formed, when all rotate together, 

 but at first only the lower or entoblastic derivatives of 4d join 

 in this rotation. In this process the ectoblast is wholly passive, 

 and the rotation of the smaller entoblasts seems to be the result 

 of purely mechanical causes (e.g., surface tension and conse- 

 quent intercellular pressure); and yet these mechanical causes 

 are governed and directed by the higher coordinating forces 

 which are at work in the building of the organism. For exam- 

 ple, the mechanical conditions would have been perfectly satis- 

 fied if the smaller entoblasts had rotated in the opposite 

 direction and had carried the ectoblastic cap with them, so that 

 as a result the planes of symmetry in the ectoblast and ento- 

 blast would not have coincided, but would have crossed each 

 other at right angles. We vmst find the ultimate cattse of this 

 anti-clockwise rotation not in such external mechanical condi- 

 tions, which are, however, incidentally fulfilled, but in those ^nore 

 complex internal conditions, wJiich direct the course of ontogefiy, 

 and which in 02ir ignorance tve call the cod7'dinati7tg force, or 

 hereditary tendency. 



5. The Four Macromeres, or Basal Quartette. Figs. J^, J/, 



^. 52. 

 At the time when the layers are all segregated the macro- 

 meres still form much the largest part of the &gg. They are 

 composed almost entirely of yolk, and their nuclei and proto- 

 plasmic portions lie near the surface just in advance of the 

 ectoblastic cap. The four cells are nearly equal in size, and 

 they are from this stage onward closely pressed together, so 

 that the egg is nearly spherical in form and never again 

 assumes the quatrefoil shape. The polar furrow extends be- 

 tween these cells from the vegetal to the animal pole, though 

 on the upper side of the egg it is covered by the cap of ecto- 



