314 



THEORIES OF INHERITANCE AND DEVELOPMENT 



to the whole must not, however, be conceived as a merely geometri- 

 cal or mechanical one ; for, in different species of eggs, blastomeres 

 may exactly correspond in origin and relative position, yet have 

 entirely different morphological value. This is strikingly shown by 



Fig. 138. — Partial larvae of the ctenophore Beroc. [DRiESCii and Morgan.] 

 A. Half i6-cell stage, from an isolated blastomere. B. Resulting larva, with four rows of swim- 

 ming plates and three gastric pouches. C. One-fourth i6-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 unsegmented egg. 

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



a comparison of the polyclade egg with that of the annelid or 

 gasteropod (Fig. 137). In both cases three quartets of micromeres 

 are successively budded off from the four cells of the four-cell 

 stage in exactly the same manner. The first quartet in both gives 

 rise to ectoderm. Beyond this point, however, the agreement ceases : 



