DEVELOPMENT OF THE INDIVIDUAL 



573 



therefore, can give rise to a complete em- 

 bryo, though smaller than normal. It seems, 

 then, that under normal circumstances only 

 a portion of this potency is utilized. When 

 the cell is retained along with its neighbors, 

 even though it has the power to become a 

 complete embryo, it merely becomes a part 

 of the embryo. There is wide variation 

 among the eggs of animals in respect to 

 potency, some being totipotent only in the 

 egg stage whereas others resemble the sea 

 urchin in this respect. 



Whether or not an egg is totipotent de- 

 pends on such factors as the presence of 

 other cells. For example, if one of the cells 

 of a frog's egg in the two-cell stage is killed 

 with a hot needle (Fig. 23-10) what will 

 happen in later development? Will it pro- 

 duce a whole or only half an embryo? 

 Because the latter is true, one might be in- 

 clined to say that the frog's egg is totipotent 

 only in the single-cell stage. If the experi- 

 ment is continued a little farther, as Hans 

 Spemann (a famous German embryologist) 

 did by tying a hair loop around the egg in 

 the two-cell stage and completely separating 

 them, each cell gives rise to a complete 

 embryo (Fig. 23-11). Strangely enough, if 

 the loop was not sufficiently tight so that 

 some part of the two cells was allowed to 

 remain in contact, a partially double em- 

 bryo resulted, that is, a pair of "Siamese 

 twins" was produced (Fig. 23-11). Un- 

 doubtedly, their counterparts in fish (Fig. 

 23-12) or humans are formed the same way. 

 For some reason, cleavage between the two 

 cells starts but is not completed, thus pro- 

 ducing embryos attached to a greater or 

 lesser degree. Occasionally one embryo 

 grows normally and incorporates the abnor- 

 mal one inside its body, forming a tumor. 

 Some of these tumors contain skin and hair, 

 indicating that they probably started as par- 

 tial twins. They often give trouble in adult 

 life and need to be removed. 



These basic concepts concerning develop- 

 ment can be carried to older embryos. How 

 soon are the cells of the embryo definitely 



Fig. 23-12. "Siamese twins" in fish. These are probably 

 formed as a result of vn incomplete separation of the 

 early embryo, perhaps in the two-cell stage. 



allocated a specific task to do, as that of 

 producing the arm, for example? By remov- 

 ing various parts of early embryos and fol- 

 lowing their subsequent development, the 

 answer to this question can be found. When- 

 ever certain parts are removed the resulting 

 embryo lacks tliose parts which that area 

 normally would have formed. If the region 

 which normally gives rise to the left arm in 

 an amphibian is removed, the embryo will 

 develop without a left arm. Furthermore, 

 if the removed part is transplanted to the 

 area of a second embryo which normally 

 produces belly, an arm will develop in the 

 new location. This means that at a specific 

 time the task of producing an arm is as- 

 signed to a certain group of cells and they 

 will produce that organ and no other if 

 placed in a position where they can be pro- 

 perly nourished. 



In the early embryo, then, every cell is 

 totipotent and can produce a complete em- 

 bryo. Later, however, potency is distributed 

 to groups of cells which can form specific 

 parts of the embryo and nothing else. This 

 sorting does not end here but continues in 

 the cell areas, becoming more and more 

 specific. For example, the arm-producing 

 region is totipotent for producing this 

 appendage, but as development proceeds 

 potency becomes more specific, that is, cer- 

 tain cells are detailed the task of producing 

 the hand, the wrist, the fingers, and so on. 

 Development proceeds from the general 

 to the specific. It might be compared to 



