EMBRYOLOGY OF CRYPTOBRANCHUS 479 



the embryo — the phenomenon of imiyiigration of cells from the 

 single-layered roof of the segmentation cavity. In a surface view, 

 it is evident that some of the cells in the excentrically situated 

 area of most rapid cell division are partially submerged. They 

 are not merely smaller superficially than the other micromeres, 

 but are sunken below the general surface and present the appear- 

 ance of being crowded inward. Their condition will be further 

 described in a consideration of the internal development; their 

 later history and fate form an important phase of the process 

 of embryo-formation. 



In most eggs of this stage, at the margin of the blastodisc 

 oblique furrows (probably fifth cleavage furrows) occasionally 

 cut off cells intermediate in size between micromeres and macro- 

 meres. In the lower hemisphere some recent furrows, presum- 

 ably fifth cleavage furrows, usually extend well toward the vicin- 

 ity of the vegetal pole. The macromeres are, as a rule, long, 

 narrow and wedge-shaped. On account of the segregation of 

 most of the protoplasm within the region of the blastodisc, all 

 latitudinal divisions of the macromeres are very unequal, cutting 

 off new micromeres instead of increasing the number of macro- 

 meres. In the living egg, the roof of the segmentation cavity 

 still appears somewhat translucent, and spaces sometimes occur 

 between these cells; but neither of these conditions is so marked 

 as in the preceding stage. 



Stage 8: (figs. 106 and 217). This stage is reached about 

 twelve hours later thkn Stage 7; it is best described by reference 

 to the figures. The micromeres have become much more numer- 

 ous and smaller; there is a slight extension of their area. There 

 is a more gradual transition, or gradation in the size of the cells, 

 between micromeres and macromeres. In the lower hemisphere 

 the fifth cleavage furrows have, as a rule, become complete; they 

 rarely reach the lower pole, but join an earlier furrow at some 

 distance from the vegetal pole. Biradiality of the cleavage pat- 

 tern still enables one, as a rule, to distinguish first and second 

 cleavage furrows in this hemisphere. 



In the upper hem' sphere the excentric area of accelerated cell 

 division noted in the preceding stages is usually quite marked. 



