380 



CELL-DIVISION AND DEVELOPMENT 



relations have, in fact, been accurately determined in a large number 

 of cases. The first to call attention to such a relation seems to have 

 been Newport ('54), who discovered the remarkable fact that the first 

 cleavage-plane in the frog s egg coincides with the median plane of the 

 adult body ; that, in other words, one of the first two blastomeres 

 gives rise to the left side of the body, the other to the right. This 

 discovery, though long overlooked and, indeed, forgotten, was con- 

 firmed more than thirty years later by Pfliiger and Roux ('87). It 



Fig. 177. Bilateral cleavage of the tunicate egg. 



A, Four-celled stage of Clavelina, viewed from the ventral side. B. Sixteen-cell stage (VAN 

 BENEDEN and JULIN). C. Cross-section through the gastrula stage (CASTLE) ; a. anterior; 

 p. posterior end; /. left, r. right side. [Orientation according to CASTLE.] 



was placed beyond all question by a remarkable experiment by Roux 

 ('88), who succeeded in killing one of the blastomeres by puncture 

 with a heated needle, whereupon the uninjured cell gave rise to a 

 half-body as if the embryo had been bisected down the middle line 

 (Fig. 182). 



A similar result has been reached in a number of other animals by 

 following out the cell-lineage ; e.g. by Van Beneden and Julin ('84) 



