28 CELL-DIVISION AND GROWTH II. i 



Before the next division the most vegetative cell (P 2 ) slips round 

 to what will be the posterior side. All four cells are bi- 

 laterally arranged about the plane in which they all lie, and 

 this will become the sagittal plane of the embryo. The anterior 

 and posterior ends, and therewith the right and left sides, are 

 likewise now determined. The bilateral symmetry is preserved 

 in future divisions, at least in the vegetative hemisphere ; in the 

 animal part of the egg the blastomeres of the left side become tilted 

 forwards, those of the right side backwards (Fig. 155, p. 255). 



4. In the Triclad Turbellarians, in Trematoda and Cestoda, 

 segmentation is irregular, the blastomeres separate from one 

 another and lie amongst the yolk-cells. The same phenomenon 

 may be witnessed in the Salps, and the separation and sub- 

 sequent reunion of the blastomeres has also been described in 

 Coelenterates and in Asteroids. 



Although these types of segmentation are distinct enough 

 from one another, intermediate conditions are readily found. 

 The radial easily passes into the spiral type for example, for 

 in many eggs of the former kind the ' cross furrows ' have been 

 observed at either one or both poles, while the animal blas- 

 tomeres may be rotated slightly on the vegetative, and so lie 

 not over, but in between, them. The radial symmetry again may 

 become bilateral, as when the meridional furrows of the fourth 

 phase, instead of passing through the animal pole, meet the first 

 or second furrow, symmetrically on either side of one of these 

 divisions ; this occurs as a variation in Sana fusca and (normally 

 (Roux)) in Sana esculenta. 



In Ophiuroids and Asteroids the tetrahedral arrangement is lost, 

 and the egg segments radially. In Amphioxus all three types occur. 



All three forms may therefore have been derived from one, 

 though what that one was we do not know. In any case, 

 however, one feature is common to them all ; in all cases suc- 

 cessive divisions are at right angles to one another. This is the 

 law formulated by Sachs long since for the divisions of the cells 

 of plants. It holds good for the segmenting animal ovum, 

 though exceptions may, of course, be found. The alternation of 

 dexiotropic and laeotropic divisions, for instance, in spirally seg- 

 menting ova continues for a long period with striking regularity, 



