26 CELL-DIVISION AND GROWTH II. i 



(Ilyanassa, Capitella, Umbrella, Crejridnla, AmpMtrite, Arenicohi, 

 for instance) the furrows are parallel, the same two blastomeres, 

 B and D, being in contact at both poles. In TrocJius both 

 the ' parallel furrows' and the 'crossed furrows' conditions 

 are found. A similar disposition is to be observed amongst the 

 micromeres of the first quartette. These micromeres, also, alter- 

 nate with the macromeres. Not more than three contact surfaces 

 between blastomeres, therefore, meet in one line. 



The eggs of certain Lamellibranchs-^-:Z<?m/0, Cyclas in which 

 the ' spiral ' arrangement is obscured by the large size of the D 

 macromere, and possibly the ova of the Rotifera, are to be referred 

 to this type. 



The tetrahedral arrangement of the first four cells is con- 

 spicuous in Asteroids and Ophiuroids, where the planes of 

 division of the first two cells are at right angles to one another. 

 Before the next division, however, the cells shift their positions 

 and come to lie in one plane, in which, however, the sister cells 

 are not adjacent, but opposite, to one another. 



The eggs of Amphioxns sometimes segment spirally (Wilson). 



After the completion of the spiral period of division, seg- 

 mentation becomes radial and then bilateral. 



3. The third type of cleavage is the bilateral. The first two 

 divisions intersect in the axis ; the next may be equatorial, as in 

 Ascidians. In this case the bilaterality becomes evident in the 

 succeeding phase, in which the divisions in two adjacent cells of 

 the animal hemisphere meet the first furrow, while in the other 

 two they meet the second. The bilaterality is marked in the 

 reverse way in the vegetative half of the egg. The egg is thus 

 divided into what will be anterior and posterior, dorsal and 

 ventral, and right and left halves. In future divisions the 

 bilateral symmetry is retained. 



The egg of Amphioxus may divide in this fashion (Wilson) , and 

 this is the normal method, according to Roux, in Rana esculenta. 



In the Teleostei and some Ganoids (Lepiclosteus) the bila- 

 terality becomes evident in the third division, which is parallel 

 to the first, the fourth being parallel to the second division. 

 The egg is in fact iso-bilateral. 



The Ctenophore egg also possesses two planes of symmetry, 



