the whole of the human race must have been present in Mother Eve! How- 

 ever, better microscopes and more thorough study have convinced most peo- 

 ple that 'preformation" does not agree with all the known facts. 



Transformation Another way of answering the paradox about chang- 

 ing and remaining the same is to recognize that the trouble may really be 

 with the words and not with the facts. A living thing is constantly chang- 

 ing, physically and chemically; and yet it remains the "same" individual. 

 The only way it can continue to be the same individual is through constantly 

 changmg. The real question is. Just exactly what changes take place be- 

 tween being an tgg and being a hen? We still have to get the facts in each 

 particular case. Just how does an tgg become transformed into a hen ? 



From Egg to Hen' Aristode was probably the first person to try to 

 answer the question How does an egg become a hen? by experimenting in- 

 stead of arguing. If we place a number of tggs under a hen (or in an 

 incubator kept at 103° F), we expect the same number of chicks to come 

 out of the cracked shells in about three weeks. We might follow Aristotle's 

 plan, removing the eggs one at a time and examining the contents. In a 

 fresh egg, even before the hatching begins, we are able to see a whitish 

 speck floating on top of the yolk— the "germ spot". Day by day this speck 

 becomes larger. In half a day, the speck is longish. Even without a micro- 

 scope we can see the beginning of structure; there is a darker line down the 

 middle (see illustration, p. 350). We are able to see more than Aristotle 

 saw, for he had no microscope. We can see in the changing chick within 

 the tgg what is perhaps more easily seen in the corresponding parts of 

 simpler animals. 



The Origin of Tissues' Eggs of frogs and of various fishes are easily 

 kept in dishes of water at ordinary room temperature. Patient watching of 

 these tggs reveals progress from the one-celled stage through several more 

 or less distinct many-celled stages (see illustration, p. 351). What we see in 

 the simpler backboned animals or in insects is similar to what we find in 

 mammals and in other classes of animals. 



In some species differences in size among the cells appear after only a 

 few divisions. In the early stages of a frog's development the cells in the 

 upper portion of the cell mass are much smaller than those in die lower 

 portion, and more numerous. 



Inequalities in the rate of division and inequalities in the growth of the 

 cells soon change the shape of the whole mass. Gradually new kinds of cells 

 appear in the young embryo. At first these are in layers, or membranes. 

 The embryos of many different species consist, at one stage, of mem- 

 branes with spaces, or cavities, among them. The membranes grow out 

 irregularly into the cavities, forming folds. They break through in some 

 iSee No. 2, p. 364, 2See Nos. 3 and 4, p. 365. 



349 



