PROCESS OF SEGMENTATION 1 9 



that the central (apical) ends of the eight cells established by 

 the third cleavage are cut off from their peripheral portions. 

 The combined contour of the fourth cleavage furrows forms a 

 small irregularly circular furrow the center of which is the point 

 at which the first two cleavage planes intersect (Fig. 5, D). 

 The central cells now appear completely separated in a surface 

 view of the blastoderm, but sections show them still unseparated 

 from the underlying yolk. 



After the fourth, the succession of cleavages becomes irregular. 

 In surface view it is possible to make out cleavage furrows that 

 divide off additional apical cells, and other, radial furrows that 

 further divide the peripheral cells. Figure 5, E and-F, show the 

 increase in number of cells and their extension out over the 

 surface of the yolk, resulting from the succession of cleavages. 

 When the process of segmentation has progressed to the stage 

 in which the succession of cleavages is irregular and the number 

 of cells considerable, the term blastoderm is applied to the entire 

 group of blastomeres formed by the cleavage of the blastodisc. 1 



In addition to the cleavages which are indicated on the sur- 

 face, at about the 32-cell stage sections show cleavage planes of 

 an entirely different character. These cleavages appear below 

 the surface and parallel to it. They establish a superficial layer 

 of cells which are completely delimited. These superficial 

 cells rest upon a layer of cells which are continuous on their deep 

 faces with the yolk. Continued divisions of the same type 

 eventually establish several strata of superficial cells. This 

 process appears first in the central portion of the blastoderm. 

 It progresses centrifugally as the blastoderm increases in size 

 but does not extend to its extreme margin. The peripheral 

 margin of the blastoderm remains a single cell in thickness and 

 the cells there lie unseparated from the yolk. 



1 While but a single spermatozoon takes part in fertilization other spermatoza 

 become lodged in the cytoplasm of the blastodisc. The nuclei of these sperma- 

 tozoa migrate to the peripheral part of the blastoderm where they are recog- 

 nizable for some time as the so-called accessory sperm nuclei. Some of them 

 appear to undergo divisions which are accompanied by slight indications of 

 division in the adjacent cytoplasm. The short superficial grooves thus formed 

 are termed accessory cleavage furrows. No cells are formed by the accessory 

 "cleavages." The sperm nuclei soon degenerate, the superficial furrows fade 

 out, and usually as early as the 32-cell stage all traces of the process have dis- 

 appeared without, as far as is known, affecting in any way the development of 

 the embryo. 



