Comparative Development. 45 



use of a large part of these tufts and they are all subsequently sup- 

 pressed, except those covered by the disc. Thus the chorion itself, 

 which is only a temporary instrument, passes through an embryonic and 

 rudimentary stage in all the deciduous mammals. 



It is interesting to observe the gradual change which takes place in 

 the period of egg-impregnation, as we ascend the scale of the vertebrate 

 animals. In most fishes the unimpregnated eggs and spermatozoa are 

 thrown into the water and allowed to find each other as it may happen. 

 Millions of eggs are thus born, a vast number of which are never im- 

 pregnated at all. In the amphibian also, the eggs are born unimpreg- 

 nated, but the parents take care to bring the elements together so that 

 impregnation is assured at the moment of the birth. In birds and rep- 

 tiles the eggs are impregnated before their birth, but in birds, as all 

 know, only a few hours before, and the entire development of the em- 

 bryo takes place after its birth. In the marsupial the egg is not only 

 impregnated but hatched before birth, but the embryo is hatched and 

 born in a very unfinished condition. It is put into the mother's pouch 

 and carried for a long time, before its development is completed. 

 Amongst the higher mammals, the period from impregnation to birth, 

 that is the period of gestation, increases as we go up the scale. 



To the question, why does the vertebrate embryo show the gill slits 

 and other marks of low stages of life, Wilson answers that it is because 

 they represent those stages as the actual ancestral stages of the verte- 

 brate possessing them ; they are legacies from the ancient state of the 

 ancestral line. I would add, that in the building of any individual 

 body, the parts can only be put together after the inherited pattern, 

 and, therefore, in the inherited sequence. The body is like a Chinese 

 puzzle, which goes together all right and easily, if the due sequence in 

 putting the parts in one after another is observed; but if it is not, and 

 any one is attempted to be placed out of its proper order, it will not fit 

 nor allow any subsequent piece to fit. If your watch has been stopped 

 an hour and you propose to set it, as you do so the hands will rapidly 

 point, in succession, to each minute that they would have indicated in 

 normal action. 



When a man does a 'thing in a round-about way, just because his 

 father did and because he learned to do so, we call him " old fogy." 

 But nature does just so in development. She is used to making a man 

 by making him first a fish, then amphibian, &c. &c, lastly, altering him 

 into a monkey and a man. It looks like a mere matter of habit with na- 

 ture, and so we shall see it is, when we properly understand what habit 

 is a question that is discussed further on. 



