448 DARWINISM 



The Embryonic Development of Man and other Mammalia. 



The progressive development of any vertebrate from the 

 ovum or minute embryonic egg affords one of the most 

 marvellous chapters in Natural History. We see the con- 

 tents of the ovum undergoing numerous definite changes, 

 its interior dividing and subdividing till it consists of a 

 mass of cells, then a groove appears marking out the median 

 line or vertebral column of the future animal, and there- 

 after are slowly developed the various essential organs of 

 the body. After describing in some detail what takes place 

 in the case of the ovum of the dog, Professor Huxley 

 continues: "The history of the development of any other 

 vertebrate animal, lizard, snake, frog, or fish tells the same 

 story. There is always to begin with, an egg having the 

 same essential structure as that of the dog; the yelk of 

 that egg undergoes division or segmentation, as it is called, 

 the ultimate products of that segmentation constitute the 

 building materials for the body of the young animal; and 

 this is built up round a primitive groove, in the floor of which 

 a notochord is developed. Furthermore, there is a period in 

 which the young of all these animals resemble one another, 

 not merely in outward form, but in all essentials of structure, 

 so closely, that the differences between them are inconsider- 

 able, while in their subsequent course they diverge more and 

 more widely from one another. And it is a general law that 

 the more closely any animals resemble one another in adult 

 structure, the larger and the more intimately do their embryos 

 resemble one another ; so that, for example, the embryos of a 

 snake and of a lizard remain like one another longer than do 

 those of a snake and a bird ; and the embryos of a dog and 

 of a cat remain like one another for a far longer period than 

 do those of a dog and a bird, or of a dog and an opossum, or 

 even than those of a dog and a monkey." 1 



We thus see that the study of development affords a test 

 of affinity in animals that are externally very much unlike 

 each other ; and we naturally ask how this applies to man. 

 Is he developed in a different way from other mammals, as 

 we should certainly expect if he has had a distinct and 

 1 Man's Place in Nature, p. 64. 



