TO THE LOWER ANIMALS 



247 



mentary to its perfect condition, runs through a series of 

 changes, the sum of which is called its Development. In 

 the higher animals these changes are extremely complicated ; 

 but, within the last half-century, the labours of such men 

 as Von Baer, Rathke, Reichert, Bischof, and Remak have 

 almost completely unravelled them, so that the successive 

 stages of development which are exhibited by a Dog, for 

 example, are now as well known to the embryologist as are 

 the steps of the metamorphosis of the silkworm moth to the 

 school-boy. It will be useful to consider with attention the 



FIG. 12. A. Egg of the Dog, with the vitelline membrane burst, so as 

 to give exit to the yelk, the germinal vesicle (a), and its 

 included spot (6). 



B. C. D. B. F. Successive changes of the yelk indicated in 

 the text. After Bischoff. 



nature and the order of the stages of canine development, 

 as an example of the process in the higher animals generally. 

 The Dog, like all animals, save the very lowest (and 

 further inquiries may not improbably remove the apparent 

 exception), commences its existence as an egg : as a body 

 which is, in every sense, as much an egg as that of a hen, but 

 is devoid of that accumulation of nutritive matter which 

 confers upon the bird's egg its exceptional size and domestic 

 utility ; and wants the shell, which would not only be 

 useless to an animal incubated within the body of its parent, 

 but would cut it off from access to the source of that 

 nutriment which the young creature requires, but which the 

 minute egg of the mammal does not contain within itself. 



