EMBRYOLOGY 1 7 3 



present time. While it took millions of years to bring about these changes, 

 the development of each individual from an apparently simple egg to 

 the visibly complex form of -the adult is now only a matter of days, or 

 even hours. The comparison may be misleading, however, since there have 

 probably been long periods when little or no change took place in the 

 species, and the next advance, appearing in a single individual, may ac- 

 tually have occurred in infinitesimal time, from gene to gene, involving 

 only a sudden alteration in one of the units of heredity. 



The identification of the egg cells with the single-celled ancestors from 

 which the higher forms have evolved calls for qualification. The converse 

 statement may be nearer the truth, namely, that the egg of today is as 

 different from the original unicellular ancestor as the adult today is dif- 

 ferent from that ancestral adult. Both statements call for reservations, for 

 everything turns on what is meant by likeness and difference. In the egg 

 there are all the potentiahties for quickly developing the characteristics of 

 the adult form, and in this sense the egg differs immensely from the 

 original one-celled ancestor. The difference lies in the units of heredity 

 in the two cases: only in their visible form are the protozoon and the egg 

 somewhat alike. Since we know nothing about the constitutional differences 

 between the hereditary elements in the original protozoon and those of the 

 egg of today, it is futile to attempt to make any serious comparisons be- 

 tween the relative complexity of the two. Only superficially are they 

 ahke in their visible structures. 



In another respect, however, we may make comparisons. The ancestral 

 type needed to pass through fewer visible changes from egg to adult. In 

 the unicellular forms, the protozoa, that multiply by self-division, each 

 daughter cell has little more to do than to enlarge to the original size, and 

 in the lower metazoa the stages, after division of the egg, are very few 

 compared with those of higher forms. But even then the comparison may 

 be misleading, for in the higher forms it is the visible changes that are 

 considered, and we think of them most often as changes in form or struc- 

 ture, while the physiological processes in the unicellular and multicellular 

 types are probably much more alike. In the higher forms these processes 

 are separated into organ systems, but they may be much the same as in 

 the protozoa. Descriptive embryology concerned itself entirely with 

 changes in form, and very little with the physiology of development. Only 

 recently has the latter received serious attention, although there have al- 

 ways been a few students interested in the physiology, especially in the 

 later stages, of the vertebrate embryo. 



For many years — let us say between 1850 and 1900 — embryologists 

 were engrossed with the idea that development of higher forms reca- 

 pitulated the entire historical path over which their evolution had passed. 

 This became known as the recapitulation theory. An immense amount of 

 purely descriptive embryological work was carried out under the in- 



