CLEAVAGE AND DEVELOPMENTAL PATTERN 573 



Although definite answers to these questions seem to be lacking, certain 

 features of Ascaris development are suggestive. There is an apicobasal 

 gradient, indicated by differential aggregation of yolk in the undivided 

 egg. From his study of dispermic eggs Boveri (igiob) concluded that the 

 polarity of the egg is represented by a cytoplasmic gradient and that nu- 

 clei coming to He in the apical levels of this gradient undergo diminution, 

 while those in the basal region do not. The behavior of dorsal and ventral 

 cells in normal cleavage suggests that the gradient differential is greater 

 in the ventral (basal) than in the dorsal (apical) half of the egg. The dor- 

 sal cell, AB, of the two-cell stage undergoes diminution as it divides into 

 A and B; the ventral cell does not. However, at the next division the cell, 

 E, evidently the more apical part of the basal half, undergoes diminution ; 

 but the cell, P2 representing the extreme basal region, does not. Evidently 

 there is sufficient gradient in the ventral (basal) half of the egg to deter- 

 mine this difference in nuclear behavior. Moreover, the dorsal cells, A 

 and B, form only part of the general ectoderm, while most of the organs 

 develop from the two ventral cells. The apical half of the egg probably 

 represents the higher levels of the apicobasal gradient, but with little axial 

 differential, most of the differential being in the basal half. The apico- 

 basal axis of the sea-urchin egg is apparently somewhat similar. If these 

 suggestions are correct, the ventral cells of the four-cell stage may deter- 

 mine the longitudinal axis. 



In formation of the rhombus some unknown factor determines the 

 change in position of the cells, at least as regards plane of the shift. In 

 tetrahedral forms this factor has apparently been somewhat less effective. 

 The factors determining the asymmetry in position of the four dorsal cells 

 from the six-cell stage (Fig. 179, E), its reversal in tetrahedral forms (Fig. 

 180, B), and the postulated relation between this asymmetry and that of 

 the adult, which concerns a single cell, are also completely unknown. 



Cleavage and development of blastomeres of two-cell and four-cell 

 stages, after killing or preventing development of others by localized radi- 

 ation, are stated always to be essentially as if the other cell or cells were 

 also developing (Stevens, 1909). The presence of the dead or inhibited 

 cells apparently does not determine the partial character of development, 

 since contact between them and the hving cells is often shght. There is no 

 conclusive evidence of reconstitution. 



Eggs kept for several months without exposure to air show various evi- 

 dences of injury in their development on exposure to air. In some all 

 chromosomes pass into one cell at the first cleavage, diminution may take 

 place at the next division, and a blastula-hke cell mass is formed, as with 



