CLEAVAGE AND DEVELOPMENTAL PATTERN 567 



basal region of this side. The number of entodermal pouches is very com- 

 monly greater than the number of macromeres present, that is, a 1/2 

 form has three, a 1/4 form two, the extra pouch being smaller than the 

 others. Driesch and Morgan regarded this as a reconstitution, but Fischel 

 holds that the extra pouch results from mechanical division of entoderm 

 by the invaginating stomodeum. 



Displacement of micromeres into two separate groups results in larvae 

 with two apical organs and four plate rows about each; but, although such 

 displacement almost always involves more or less displacement of macro- 

 meres, the larvae are single as regards entoderm and stomodeum. Isola- 

 tion experiments agree in general in showing that plate-row-forming cells 

 or cell groups are so far determined at an early stage that isolation does 

 not usually alter their course of development. The most interesting point 

 is the usual absence of reconstitution of plate rows in the partial forms. 

 The adult ctenophore shows high capacity for reconstitution of plate rows 

 from general ectoderm, as well as of other parts;'-' consequently, the gen- 

 eral ectoderm must be potentially capable of giving rise to plate rows. 

 Perhaps developing plate rows exercise some degree of dominance over the 

 general ectoderm, and this may be adequate in the small larval forms to 

 inhibit reconstitution of other rows. Since the plate rows are undoubtedly 

 regions of more intense physiological activity than the general ectoderm, 

 they probably influence it in some way. With increase in size of the indi- 

 vidual this dominance may not extend over the whole ectoderm between 

 rows, and reconstitution of other rows becomes possible. The fact that 

 partial forms sometimes develop more, sometimes fewer, plate rows than 

 expected from the number of micromere groups present is of interest in this 

 connection, as suggesting such relations. Also of interest are the observa- 

 tions of Chun (1892, 1895) on 1/2 larvae of Bolina found after a storm 

 which had presumably isolated blastomeres. These forms had developed 

 to larvae as 1/2 forms, but on metamorphosis they reconstituted to 

 wholes. 



Except as regards the plate rows, the forms resulting from isolation and 

 displacement of blastomeres show considerable reconstitution. Duplica- 

 tion of apical organs and arrangement of plate rows about the duplicated 

 organs involves change in position and direction of some of the plate rows, 

 if not actual reconstitution. In 1/2, 1/4, and 1/8 forms ectoderm over- 

 grows the entoderm completely ; that this is entirely a mechanical matter 

 seems doubtful. Also, the stomodeum in such forms is a whole, not a par- 



'^ Mortensen, 1913; Coonfield, 1936a, h, 1937a, b. 



