4i8 



ISIIERITAXCE AXD DEVELOPMEXT 



~\Ve now proceed to the consideration of experiments which show- 

 that in some animal egK'^ ^^»ch ]-)redetermination may go much farther, 

 so that the development does, in fact, show many of the features of 

 a mosaic-work, as maintained ]:>y Roux. The best-determined of these 

 cases is that of the ctenoph()re-e«;K. ^^^ shown by the work of Chun, 



Fig. 189. — Partial larvcne of the ctenophore Beroc. [Dkiesch and Morgan.] 



A. Half sixteen-cell stage, from an isolated blastomere. li. Resulting larva, with four rows of 

 swimming-plates and three gastric pouches. C. One-fourth sixtecn-cell stage, from an isolated 

 blastomere. D. Resulting larva, with two rows of plates and two gastric pouches. E. Defective 

 larva, with six rows of plates and three gastric pouches, from a nucleated fragment of an unseg- 

 mented egg. F. Similar larva with five rows of plates, from above. 



Driesch, and Morgan ('95), and Fischel ('98). These observers have 

 demonstrated that isolated blastomeres of the two-, four-, or eight-cell 

 stage undergo a cleavage which, through the earliest stages, is exactly 

 like that which it would have undergone if forming part of a com- 



