244 



INTERNAL FACTORS 



IV. i 



ever, gastrulate ; when once the gastrula stage has been reached, 

 therefore, further development may be simply a matter of 

 sufficiency of substance. So again the octants of the Ctenophore 

 egg are alike, but each can produce one costa, and one only. 



There are then certain preformed organ-producing substances, 

 and the arrangement which they either possess before, or acquire 



FIG. 149. Three segmentation stages in the blastoderm of Sepia offici- 

 nalis; the segmentation is of the bilateral type. I, left; r, right; I-V, 

 first to fifth cleavages. The top sides of the figures are anterior. (After 

 Vialleton, from Korschelt and Heider.) 



during, segmentation determines the fixed relation which can be 

 observed in almost all cases between the structure of the egg and 

 the axes of the embryo. Thus in Amphibia the head of the 

 embryo is formed at or near the animal pole, and the plane of 

 egg symmetry becomes the sagittal plane. In Mollusca (not 

 Cephalopoda) and Annelida the apical sense-organ is developed 

 at the animal pole, while the D quadrant is posterior. A blasto- 



