single cell becomes in a few days a spherical mass of many more or less 



similar cells. 



In three weeks the mass of cells has become elongated, but hardly recog- 

 nizable as any particular "kind" of animal, although it has distinct vertebrate 

 characteristics. There are many kinds of cells. The surface, or "skin", cells 

 differ from the internal cells. Certain layers come to be more like "muscle", 

 and others come to be definitely digestive structures. It is possible to see 

 little knobs of cells that correspond in position and form to prospective 

 "bone" masses. Other lumps of cells suggest the beginnings of nerve tissue. 



By the end of the fourth week there can be no doubt that the young 

 embryo is a mammal, and not a fish or a bird. At five weeks, little buds in- 

 dicate the positions of arms and legs. Later the tips of these buds begin to 

 divide into the rudiments of fingers and toes. While the head end of the 

 embryo has in the meantime been growing faster than other parts, we could 

 hardly recognize the features as being especially "human" until about a 



month later. . 



Now the eyes and ears and nose and chin become steadily more distinct 

 —and more distinctly human. By the time the baby is born, it is already a 

 particular person.- In every family those who see the young infant usually 

 remark upon its resemblance to one or another of its various relatives. One 

 observer sees the mother's eyes or the father's mouth. Somebody else recog- 

 nizes an aunt's chin or a grandparent's forehead. In other words, that in- 

 distinguishable cell or lump of cells has come to be not only a human being 

 but a unique human being, a distinct combination of organs and features 

 and tissues and chemical characteristics that is different from any other living 

 combination. And at the same time, not only does this human being con- 

 sist of the "same" kinds of organs and tissues and processes as other human 

 beings and other mammals, but it has passed through the "same" distmct 

 stages of development as other backboned organisms (see illustration, 



P-459). .^ 1 r u 



Recapitulation The foundations for the scientific study of embry- 

 ology were laid by Karl Ernst von Baer (1792-1876), who was born of Ger- 

 man parents in Estonia, but was educated in Germany, where he did most 

 of his work, spending the latter part of his life in Russia. Von Baer was the 

 first to work out the development of the hen's egg layer by layer, so to say. 

 He was also the first to see the original egg cell in a mammal, in 1827, 

 twelve years before the form.ulation of the cell theory. 



In comparing all the embryos that he could study, von Baer was im- 

 pressed by the corresponding stages of development among different species. 

 This uniformity has been called von Baer's "biogenetic law"-a general 

 description of what we can observe in the development of many kinds of 

 eggs into adult animals. Half a century later some biologists expanded this 



356 



