CHAPTER I 

 INTKODUCTION 



THE science of Embryology has for its subject-matter the growth of 

 animals from the time when they first appear as germs in the bodies 

 of their parents until they reach the adult condition, and are 

 able to produce similar germs themselves. It thus includes a study 

 of the complete life-cycle, and is much more extensive in its scope 

 than ordinary descriptive Comparative Anatomy which confines itself 

 to a study of the adult forms. 



In practice, however, the study of the adult form precedes the 

 study of all other stages of the life-history, because it is assumed that 

 in the adult producing ripe germs, we have a stage which is the same 

 whatever kind of animal we examine. A text-book of Embryology 

 therefore assumes a knowledge of the adult forms of the animals 

 whose life-histories it describes. Even with this limitation the scope 

 of Embryology would be enormous were it not for a defect which is 

 often overlooked, but which renders it possible to bring the most 

 important results before the reader within moderate compass ; 

 this defect is the extreme difficulty of finding out with any com- 

 pleteness the whole course of any given life-history. 



The life-history of an animal is only in the rarest cases directly 

 observed ; it is deduced from a comparison with one another of in- 

 dividuals of various ages, and only when we can examine a large number 

 of individuals belonging to stages separated from one another by very 

 short intervals can we get any reliable results. Numberless mistakes 

 have been made in the past and will continue to be made in the 

 future, by the effort to re-construct a life-history from the observation 

 of an insufficient number of stages. Thus, to give examples, the 

 germ cells of Balanoglossus and of the Annelid Lopadorliynchus have 

 been stated to arise from the ectoderm, whereas in reality in both cases 

 they arise from the lining of the body -cavity or coelom. Sometimes the 

 life-history has been actually read backwards : thus the later stages in 

 the Toruaria larva have been regarded as the earlier. In the case of 

 the vast majority of animals only bits and scraps of the life-history 

 are known, and the number of cases in which the whole course of 

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