35 INTRODUCTION. 



set forth connectedly from beginning to end. It is in this way that 

 KO'LLIKER'S " Embryology of Man and the Higher Animals" is written. 



The second method is, moreover, the only one applicable when the 

 problem is to investigate in a comparative way the development of 

 several organisms, and to fill up the gaps which exist in our know- 

 ledge of one by that which we know concerning nearly related 

 animals. But it is precisely in this position that we find ourselves 

 when we wish to acquire a survey of the development of the human 

 body. An account which should limit itself to that which we know 

 about Man would exhibit numerous and extensive gaps. For up to 

 the present the eye of man has not seen how the human ovum is 

 fertilised, how it divides, how the germ-layers are formed, or how 

 the establishment of the most important organs is effected. It is 

 especially the period of the first three weeks, during which the 

 greatest variety of fundamental processes of development take place,, 

 concerning which we know next to nothing ; there is also little 

 prospect that a change will soon occur in this regard. The time 

 will therefore perhaps never come when a complete embryology of 

 Man in the strict sense of the word will be possible. 



However, the existing gaps can be filled out in another manner r 

 and one which is entirely satisfactory. The study of the most widely 

 differing Vertebrates teaches us that they are developed according 

 to a common plan, that the first processes of development agree 

 in all really important points, and that the differences which we 

 encounter here and there are produced by causes of a subordi- 

 nate kind, as, e.g., by the egg's possessing a greater or less amount 

 of yolk. 



When we see that the establishment of the central nervous system, 

 of the eye, of the spinal column, of the viscera, etc., takes place in 

 Mammals on the whole just as it does in Amphibia, Birds, and 

 Reptiles, the conclusion is near at hand, and justified, that Man 

 also in his development is no excf pt ; on to this general phenomenon. 

 Thus in the study of Embryology we are naturally led to the com- 

 parative method. What, owing to the nature of the difficulties, we 

 cannot learn directly about the development of Man, we seek to 

 deduce by the investigation of other Vertebrates. 



In earlier decennia the Hen's egg was the favorite object, and it 

 is upon this that we possess the most numerous and most complete 

 M'rit s of observations. During the last twenty years research lm> 

 !>een directed to Mammals, in the investigation of which tho 

 tlillicultifs have to be surmounted, :is well as to Kq, tiles, 



