284 



GENERAL EMBRYOLOGY 



Ascidian, where one of four, eight, or sixteen cells gives rise, not 

 to a complete diminutive larva, but to a group of differentiated 

 tissue cells of the same kind that would normally have been 

 formed from the particular cell, had it remained in situ in the 

 normal group. 



The discordance of these results may have one of two mean- 

 ings. First, it may mean that in such eggs as those of the 

 Echinoderms and Amphioxus a process of regeneration or 



C D E 



FIG. 135. Cleavage in the egg of the sea-urchin, Echinus micro-tuberculatus, 

 under pressure. From O. Hertwig, after Ziegler. A, B. Eight- and sixteen-cell 

 stages. C. Sixteen-cell stage preparing for division. D. Thirty-two-cell stage, 

 in the form of a flat plate. E, Thirty-two-cells preparing for next division. 

 Crosses mark cells in which the spindle is vertical or oblique, to the plane of the 

 cell group. 



regulation goes on. And, just as many adult organisms are 

 easily capable of restoring or regenerating lost parts, so the 

 embryo or even the ovum may have the property of reforming 

 parts artificially removed. This process has received the 

 special term of post-generation (Roux). Such a possibility is 

 indicated by the classic experiment of Roux upon the egg of the 

 frog. Here, if one of the two blastomeres is destroyed the 



