566 PATTERNS AND PROBLEMS OF DEVELOPMENT 



tion to polar axis is not certainly known or can be inferred only from de- 

 velopmental results, but in eggs beginning first cleavage and later stages 

 definite orientation of plane of section is possible. After removal of an 

 apical portion by section more or less transverse to the apicobasal axis, de- 

 velopment differs according to amount and region removed. Removal of 

 parts by oblique section basal to the equator usually results in larvae with 

 one or more plate rows defective or lacking; but after removal of parts by 

 section apical to the equator, normal larvae may develop. After lateral re- 

 moval by section parallel to the apicobasal axis one or more plate rows 

 are usually defective or absent. Defects are greater after formation of po- 

 lar bodies than before (Yatsu). From the experimental data Fischel con- 

 cluded that plate-row material is localized in a ring near the basal pole, but 

 Spek regards regional differentiation of the ectoplasm before or at begin- 

 ning of first cleavage as improbable and maintains that defective develop- 

 ment or absence of one or more plate rows results from removal of a cer- 

 tain amount, rather than a particular kind, of ectoplasm. Since the ecto- 

 plasm aggregates basally before the first cleavage, removal by section of 

 cytoplasm about this region will remove a relatively large amount of ecto- 

 plasm and so interfere later with development of some of the micromeres. 

 The defective development of one or more plate rows following removal of 

 peripheral cytoplasm basal to the equator seems to indicate that the ecto- 

 plasm does actually migrate apically, as Spek asserts, for the plate rows 

 develop about the apical region. 



Cleavages of isolated blastomeres, as far as followed, occur always as if 

 the other cells were present. Each 1/2 blastomere gives rise to four plate 

 rows, each 1/4 blastomere to two, each 1/8 to one. According to Yatsu, 

 the end cells and the middle cells of the eight-cell state (Fig. 177, 5) ap- 

 parently differ in some way, for in groups of middle blastomeres the num- 

 ber of plate rows developing is often less than the number of middle cells 

 present ; but isolated single or paired end cells give rise, respectively, to 

 one or two rows. In groups including both middle and end cells the num- 

 ber of plate rows developing is sometimes greater than the number of 1/8 

 blastomeres present, suggesting some reconstitution. Groups of 1/16 

 blastomeres, including more of the micromeres than of the macromeres, 

 usually develop plate rows equal in number to the micromeres, but some- 

 times fewer (Fischel, Yatsu). 



The ectoderm of these partial forms incloses the entoderm completely ; 

 but the side of the larva representing the original surface of contact with 

 other cells remains flattened, and the stomodeum invaginates from the 



