342 THE FORMS OF TISSUES [ch. 



in molluscs and annelids ' stands in obvious relation to the different 

 size of the cells produced at the two poles*.'" 



When the two lateral bubbles are gradually reduced in size, 

 or the two terminal ones enlarged, the upper furrow becomes 

 shorter and shorter; and at the moment when it is about to 

 vanish, a new furrow makes its instantaneous appearance in a 

 direction perpendicular to the old one; but the inferior furrow, 

 constrained by its attachment to the base, remains unchanged, 

 and accordingly our two polar furrows, which were formerly 

 parallel, are now at right angles to one another. Instead of a 

 single plane quadrilateral partition, we have now two triangular 

 ones, meeting in the middle of the system by their apices, and 

 lying in planes at right angles to one another (Fig. 135, 5-7) f. 

 Two such polar furrows, equal in length and arranged in a cross, 

 have again been frequently described by the embryologists. 

 Robert himself found this condition in Trochus, as an occasional 

 or exceptional occurrence: it has been described as normal in 

 Asterina by Ludwig, in Branchipus by Spangenberg, and in 

 Podocoryne and Hydractinia by Bunting. It is evident that it 

 represents a state of unstable equilibrium, only to be maintained 

 under certain conditions of restraint within the system. 



So, by sUght and delicate modifications in the relative size of 

 the cells, we may pass through all the possible arrangements of the 

 median partition, and of the "furrows" which correspond to its 

 upper and lower edges ; and every one of these arrangements has 

 been frequently observed in the four-celled stage of various embryos. 

 As the phases pass one into the other, they are accompanied by 

 changes in the curvature of the partition, which in like manner 

 correspond precisely to phenomena which the embryologists have 

 witnessed and described. And all these configurations belong to 

 that large class of phenomena whose distribution among embryos, 

 or among organisms in general, bears no relation to the boundaries 

 of zoological classification ; through molluscs, worms, coelenter- 



* E. B. Wilson, The Cell-lineage of Nereis, Journ. of Morphology, vi, p. 452, 

 1892. 



t It is highly probable, and we may reasonably assume, that the two little 

 triangles do not actually meet at an apical point, but merge into one another by 

 a twist, or minute surface of complex curvature, so as not to contravene the normal 

 conditions of equihbrium. 



