532 - Heredity and Evolution 



effects, owing to small differences in the com- 

 position of the surrounding cytoplasm. Even 

 at the time of fertilization there are always 

 significant differences in cytoplasmic com- 

 position at the poles of the zygote. Moreover, 

 the cytoplasm of the animal pole goes mainly 

 to an upper group of cells, during normal 

 cleavage (Fig. 15-2); and the cytoplasm of 

 the vegetal pole goes to the lower cells. In- 

 evitably, therefore, differences occur between 

 the cells at a very early stage of development. 

 And as development continues, many other 

 differentials develop in the genie environ- 

 ments of the various regions of the embryo. 

 The chemical environment is not uniform, 

 since some cells come to lie in a more advan- 

 tageous position with reference to the supply 

 of oxvgen and other nutrients, and some 

 cells have an easier problem in disposing of 

 their metabolic wastes. Substrates present in 

 one cell may become different from those in 

 another, and consequently different enzymes 

 may be induced to form. Also, the distribu- 

 tion of physical factors — such as pressure 

 (from other cells), light, heat, and electric 

 fields — do not impinge uniformly upon the 

 different cells. All these chemical and physi- 

 cal forces appear to cooperate in determining 

 the differential action of the genes in the 

 different cells of the embryo. One main prob- 

 lem in experimental embryology is, therefore, 

 to determine which of these environmental 

 factors play crucial roles in the differentia- 

 tion of particular structures, and when this 

 control is exerted in the case of each differ- 

 ent kind of cell. 



Pressure from neighboring parts of the 

 embryo may greatly influence developmental 

 processes, as is demonstrated by a classic ex- 

 periment on the frog's egg. If the two blasto- 

 meres are separated in the two-cell stage of 

 development, each of the separated cells de- 

 velops into a small but perfectly whole em- 

 bryo. But if one of the two cells is merely 

 killed and left in contact with its companion, 

 the remaining cell develops into only half an 

 embryo. This shows that pressure, or at least 

 some other factor associated with the con- 



tiguous cell, dead though it be, has had a 

 very profound influence upon the develop- 

 mental destiny of all the other cells of the 

 embryo. 



Some Problems of Morphogenesis. A num- 

 ber of investigators are currently attempting 

 to analyze the various factors and forces that 

 operate to determine the form and arrange- 

 ment of the aggregated cells of the embryo 

 during development. Here, however, it is 

 possible to give only a very brief account of 

 results obtained by T. Gustafson of Stock- 

 holm and L. Wolpert of London (working 

 in collaboration) and by J. T. Bonner 

 (Princeton University), B. Shaffer (Cam- 

 bridge University), and A. A. Moscona (Uni- 

 versity of Chicago). 



Wolpert and Gustafson in their time-lapse 

 cinematographic studies of morphogenesis 

 in sea urchin embrvos have emphasized the 

 importance of four simple factors: (1 ) changes 

 in the areas of cohesion between contiguous 

 cells; (2) changes in the areas of adhesion be- 

 tween the cells and some supporting mem- 

 brane; (3) pseudopodial activity; and (4) 

 adhesion between the pseudopodia and sta- 

 tionary cells that are contacted by them. 



An indication of how the first two factors 

 (cohesion and adhesion areas) ma}' operate is 

 given in Figure 27-6. Furthermore, it is not 

 difficult to visualize how the other two fac- 

 tors may come into plav. Shortly after a 

 blastula is formed, a number of cells lose 

 contact with the supporting membrane at 

 the surface of the embryo. These cells then 

 become crowded into the blastocoel, where 

 they display vigorous pseudopodial activity. 

 The pseudopodial out-thrustings appear to 

 be random. However, there are certain areas, 

 bordering the blastocoel, where the amoeboid 

 cells, or rather their pseudopodia, tend to 

 stick. By a contraction of the pseudopodia, 

 therefore, the migratory amoeboid cells may 

 come into close association with certain cells 

 on the surface of the blastula. In other words, 

 localized changes in the cohesive and adhe- 

 sive properties of cells, together with cellu- 

 lar motility, are important factors in deter- 



