THE CONTINUITY OF LIFE 



completely will their embryonic development coincide, and 

 conversely, in forms widely apart the divergence begins very 

 early and only the first of the two developmental histories will 

 be coincident. 



To illustrate those points: if the development of 16 and 34 

 be compared, only the early stages I, 2 and 3 will be seen to 

 coincide; if, however, the developmental histories of 21 and 

 1 6 be taken, they will be found coincident as far as their last 

 common ancestor, 5. In closely allied forms, such as 37^ and 

 37g, almost the entire embryological history in the two animals 

 will closely correspond, differences being noted only at the last. 



IV. The more highly specialized the animal, the more 

 changes its ancestors have passed through; and therefore so 

 much the more is to be recapitulated onto genetically. This is 

 effected in part by lengthening the embryonic period and in 

 part by sliding over or dropping out some of the stages. 



In the fish, for example, after the development of a simple 

 circulation designed for a water-breathing vertebrate, there is 

 nothing farther to do than to perfect and to mature it as it is; 

 in the mammal, however, the circulatory system, which is at 

 first like that of the embryonic fish, must become successively 

 modified as amphibian, reptilian, and finally mammalian; a 

 much longer history, which involves numerous changes and 

 adaptations. 



V. The different historic stages are not given the same time 

 value, but the earlier the stage, the more it is accelerated. 

 The earlier stages also lose in distinctness and detail and are 

 more often lost than the later ones. It follows from this that 

 the early part of the history is best learned from the lower 

 forms, in which the stages sought are not very remote from 

 the adult condition. 



The approximate time values of the developmental stages 

 are seen in the development of the hen's egg ; the segmentation 

 stages, and the formation of blastula and gastrula, which rep- 

 resent all the earlier invertebrate portion of the history, are 

 passed through in a few hours ; the establishment of the meso- 

 dermic somites (myomeres), which makes it a vertebrate, is 



