24 HISTORY OF THE HUMAN BODY 



well marked by the end of the second day ; at the age of four 

 days the embryo is sauropsidan, at five or six definitely avian, 

 and the remaining fifteen days are spent in perfecting the 

 details first of a gallinaceous bird, and lastly of the particular 

 species to which it belongs. Furthermore, the remainder of 

 the history, until the adult stage is reached, that is, the latest 

 historical period, requires many months. The value of the 

 study of the more primitive forms is well seen by the forma- 

 tion of the mesoderm, and especially that part of it which 

 give rise to the myomeres or primitive muscle segments. In 

 Amphioxus, a form considerably below the fishes, the mesoderm 

 arises from the primordial intestine in the form of paired di- 

 verticula, from the dorsal part of which the myomeres arise; 

 in fishes and amphibians these elements are not distinct 

 diverticula, but still possess cavities or the rudiments of them ; 

 and in birds and mammals the myomeres arise as solid cubes 

 cut from an indifferent cell mass, and give absolutely no clew 

 to their early history. 



VI. In studying an embryological record one must con- 

 stantly distinguish between palin genetic characters, or those 

 which are true repetitions of the past history, and cccnogenetic 

 characters, or those which have been more recently acquired 

 as the result of some special adaptation. One of the most 

 universal among these latter is the presence of yolk, a food 

 supply for the embryo, which lies between or within the cells 

 and, when excessive, causes misleading distortions in the pro- 

 portion of parts and effects the obliteration of many important 

 features. 



In general the actual size of an egg is due to the amount 

 of yolk it contains, and thus the historic records are reproduced 

 with greater faithfulness in very small ones. This is well 

 shown by the comparison of the almost yolkless egg of Am- 

 phioxus with that of the bird, which represents the other 

 extreme. In the one the cylindrical form of the primitive 

 vertebrate is well preserved and appears almost at the begin- 

 ning; in the other the dorsal portion of the future body lies 

 for a time almost flat on the surface of an enormous sphere 

 of yolk, and is enabled later to assume the cylindrical form 



