VII] OF POLAR FURROWS 559 



and shorter; and at the moment when it is about to vanish, a new 

 furrow makes its instantaneous appearance in a direction perpen- 

 dicular to the old one; but the inferior furrow, constrained by its 

 attachment to the base, remains unchanged, and it looks as though 

 our two polar furrows, which were formerly parallel, were now at 

 right angles to one another. But in fact, the geometry of the 

 whole system is entirely altered. Before, two furrows left each 

 end of one polar furrow for the same end of the other polar furrow, 

 and the two cells at either end were shaped like "liths" of an orange. 

 Under the new arrangement, two furrows leave each end of one for 

 the two ends of another. The figure is now divided by six similar 

 furrows into four similar curviHnear triangles*; it has become 

 (approximately) a spherical tetrahedron, and the four cells into 

 which it is divided are four similar and symmetrical figures, also 

 tetrahedral, all meeting in a point at the centroid of the figure. 

 Such a four-celled embryo, described as having two polar furrows 

 arranged in a cross, has often been seen and figured 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. 



So, by slight and delicate modifications, we pass through many, 

 and perhaps through all, of the possible arrangements of external 

 furrows and internal partitions which divide the four cells from one 

 another in a four-celled egg or embryo; and many, or most, or 

 possibly all of these arrangements have been more or less frequently 

 observed in the four-celled stages of various embryos. iVnd all 

 these configurations, which the embryologists have witnessed and 

 described, 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, coelenterates, vertebrates and what not, we meet with now 

 one and now another, in a medley which defies classification. They 

 are not ''vital phenomena," or "functions" of the organism, or 

 special characteristics of this organism or that, but purely physical 



* That the sphere can be symmetrically divided into four equilateral triangles, 

 after the manner of these embryos (or of many pollen-grains), is an elementary 

 fact of great importance in geometry and trigonometry. 



