6 ON EMBRYOLOGY* 



Embryology, may be regarded as little more than an ex- 

 tension, with corrections, of his observations. Were it de- 

 sirable to re-publish Von Baer's work, the corrections and 

 expansions of matters of fact necessary to bring it up to the 

 present time, as the phrase goes, would, with some few 

 exceptions, be of minor importance, though they might be 

 many. The theoretical considerations embodied in his 

 Scholia through which he interprets the morphological sig- 

 nificance of embryological facts are of great and lasting 

 importance, though they need some modifications in order to 

 bring them into harmony with the theory of natural selection. 

 Since Von Baer's time, the advances made in Vertebrate Em- 

 bryology, through the elaborate work of Remak, the labours of 

 Eathke, Allen Thomson and others, the admirable lectures of 

 Kolliker, and the researches of more recent inquirers, though 

 many and varied, cannot be said to constitute any epochs in 

 the history of the subject, such as that which was marked by 

 Von Baer, and before him by Wolff. We may perhaps make 

 an exception in favour of the discovery by Purkinje, of the 

 germinal vesicle in the fowl's ovarian ovum (1825). This led 

 to Von Baer's discovery of the mammalian ovum (1827), which 

 first rendered possible a consistent view of mammalian gene- 

 ration. 



The study of invertebrate embryology has, 'on the other 

 hand, during the last few years produced the most striking 

 results. 



In the following pages we propose to follow in the path 

 thus marked out by the history of the subject. We begin 

 with the chick as being the animal which has been most 

 studied, and the study of which is easiest, and most fruitful 

 for the beginner. The first part accordingly will be devoted 

 to a description of the changes undergone by an incubated 

 hen's egg, especially during the early days of incubation. We 

 shall endeavour to explain, with such details as are necessary, 



