4i8 



IMIERITAXCK AXP DEVELOPMENT 



- We now proceed to the consideration of experiments which show 

 that in some animal eggs such jiredetermination may go much farther, 

 so that the develoi:)mcnt does, in fact, show many of the features of 

 a mosaic-work, as maintained by Roux. The best-determined of these 

 cases is that of the ctenophore-egg, as shown by the work of Chun, 



Fig. 189. — Partial larvae of the ctenophore Berce. [DriesCH and MORGAN.] 



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

 swimming-plates and three gastric pouches. C. One-fourth sixteen-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- 



