70 REPRODUCTION AND LIFE HISTORY OF ANIMALS. 



our ancestors. Embryologists have made this fact most 

 vivid, by showing that the individual animal develops along 

 a path the stations of which correspond to some extent with 

 the steps of ancestral history. 



(1) The simplest animals are single 



cells (Protozoa). 



(2) The next simplest are balls of 



cells (e.g.) Volvox}. 



(3) The next simplest are two- 



layered sacs of cells (e.g., 

 Hydra}. 



(1) The first stage of development 



is a single cell (fertilised 

 ovum). 



(2) The next is a ball of cells 



(blastula or morula). 



(3) The next is a two layered sac 



of cells (gastrula). 



Von Baer, one of the pioneer embryologists, acknow- 

 ledged that with several very young embryos of higher 

 Vertebrates before him, he could not tell one from the 

 other. Progress in development, he said, was from a general, 

 to a special type. In its earliest stage, every organism has 

 a great number of characters in common with other 

 organisms in their earliest stages ; at each successive stage 

 the series of embryos which it resembles is narrowed. The 

 rabbit begins like a Protozoon as a single cell, after a while 

 it may be compared to the young stage of a very simple 

 vertebrate, afterwards to the young stage of a reptile, after- 

 wards to the young stage of almost any mammal, afterwards 

 to the young stage of almost any rodent, eventually it 

 becomes unmistakably a young rabbit, 



Herbert Spencer expressed the same idea, by saying that 

 the progress of development was from homogeneous to 

 heterogeneous, through steps in which the individual history 

 was parallel to that of the race. But Haeckel has illustrated 

 the idea more vividly, and summed it up more tersely than 

 any other naturalist. His "fundamental biogenetic law" 

 reads, " Ontogeny, or the development of the individual, is a 

 shortened recapitulation of phylogeny, or the evolution of 

 the race." 



It is hardly necessary to say that the young mammal is 

 never like a worm, or a fish, or a reptile. It is at most like 

 the embryonic stages of these, and it may also be noticed 

 that as our knowledge is becoming more intimate, the 

 individual peculiarities of different embryos are becoming 

 more evident. Thus Professor Sedgwick has recently said 

 that a blind man could distinguish the early stages of 



