CLEAVAGE AND DEVELOPMENTAL PATTERN 571 



observed to form at the blunt pole in about three-fourths, elsewhere in 

 others. Most of these eggs developed abnormally, but in 91 per cent of the 

 normal cleavage stages polar bodies were at the blunt pole (Schleip, 1924). 

 After fertilization and maturation the yolk becomes more densely aggre- 

 gated basally, so that an apicobasal gradient is visible in the cytoplasm. 



The cleavage pattern of Ascaris and of various other nematodes studied 

 differs from other known patterns. The first cleavage is transverse to the 

 apicobasal axis and is practically equatorial (Fig. 179, A), the apical cell, 

 AB, being regarded as dorsal, the basal cell, Pi, as ventral ; the two cells are 

 different, Pi containing more yolk. The second cleavage is meridional in 

 relation to the egg axis and is accompanied by diminution of chromatin in 

 the dorsal AB, transverse and without diminution of chromatin in the 

 ventral Pi (Fig. 179, B). A change in position of blastomeres follows this 

 cleavage (Fig. 179, C), resulting in a "rhombus" form (Fig. 179, D). Up 

 to this stage it has not been possible to distinguish future anterior and pos- 

 terior ends; but in the rhombus P. and the cell with which it comes into 

 contact {B) are posterior, the other two cells anterior, and the nuclei of 

 all four cells are in the median plane. According to Boveri, A and B give 

 rise to general ectoderm, largely of the anterior and dorsal regions; EMS 

 forms entoderm, mesoderm, and stomodeum; and P^, ectoderm, meso- 

 derm, and germ cells. Evidently there is no segregation of embryonic 

 layers except entoderm in particular cells. At the next cleavage A and B 

 divide equally in the median plane; but the four resulting cells shift to 

 somewhat oblique positions, so that in lateral view the right anterior cell 

 of the four is higher than the left anterior, producing a slight asymmetry 

 (Fig. 179, £). The cell EMS divides into posterior (entoderm) and ante- 

 rior (mesoderm and stomodeum) cells, both of which undergo chromatin 

 diminution. The spindle of this division is indicated in Figure 179, E. 

 P2 also divides into C and P3 without diminution, but at the next division 

 C undergoes diminution, giving rise later to ectoderm. 



With change in position of cells of the four-cell stage in a direction at 

 right angle to the usual direction, a tetrahedral form (Fig. 180, ^), instead 

 of a rhombus, results. Intermediate directions of the shift occur, but 

 finally result in either rhombus or tetrahedral form. The tetrahedral form 

 supposedly results from injurious or inhibiting conditions; high tempera- 

 ture increases its frequency (Bonfig, 1925). In this form the two dorsal 

 cells divide transverse to the axis of the two ventral cells, that is, at right 

 angles to their divisions in the rhombus form ; and the cell on the left side 

 and anterior with respect to the ventral cells is higher in lateral view (Fig. 



