140 



EMBRYOLOGY 



Fig. 65 0, each cleavage sphere is divided into a larger and 

 a smaller part. The most striking thing in this is that by 

 the regular paired arrangement of the four smaller cleavage 

 spheres a difference between the cross-axes (secondary axes) 

 of the embryo can already be recognized, so that even as 

 early as this stage the biradiate structure is clearly ex- 

 pressed ; and, according to FoL (No. 7), the longer of the 

 two diameters corresponds to the transverse, and the shorter 

 to the sagittal, axis. The transverse plane (infundibular or 

 tentacular plane) therefore separates the embryo into two 



Fis. 65.— Diagrammatic representation of the cleavage of the Ctenophora, based 

 on the figures of A. Agassiz. A, stage of division into two cells; B, fonr-cell 

 stage from the side ; C, eight-cell stage seen from above; D, the same in transverse 

 section ; E, two-cell stage from above ; F, four-cell stage from above ; G, plan of 

 the next succeeding division ; H, transition to sixteen-cell stage ; I, the same from 

 above; K, L, succeeding stages with multiplication of the micromeres; M, such a 

 Btage in cross-section. 



rows of four cells each, as is represented in Fig. 65 C. 

 Another peculiarity of this stage consists in the fact that 

 its eight cells no longer lie in one plane ; the smaller 

 lateral cells move to a higher level, whereby, as can be 

 seen in Fig. 65 C and 7), the entire fundament becomes 

 somewhat basket-shaped. In this way a difference be- 

 tween the two poles of the chief axis is also indicated 

 even now, and the concavity of the basket-like fundament 



