238 



INTERNAL FACTORS 



IV. i 



Conklin has also divided the gastrula into anterior and posterior 

 halves ; neither can regenerate the missing parts, though the cut 

 surface of each becomes covered by ectoderm. 



Driesch, however, states that in another form, Phallusia 

 mammillata, anterior and posterior half gastrulae will, if 



FIG. 146. Cynthia partita. Anterior half and three-quarter embryos. 

 (After Conklin.) a, b, anterior ventral three-quarter embryo, the 

 posterior dorsal cells (B4.1, B 4.1) containing all the yellow pigment 

 having been killed in the 8-cell stage, a, ventral view ; 6, dorsal view. 



c, d, anterior half embryo : c, dorsal view, showing the neural plate 

 (stippled) ; d, deeper focus showing two rows of chorda cells (the nuclei 

 are shaded), and ectoderm and endoderm. 



separated before complete closure of the blastopore, each develop 

 into a larva lacking only the otolith and the eye. In a later 

 stage of gastrulation this becomes impossible, although the 



