TO THE LOWER ANIMALS. 75 



pillar ; and each of these beings, in passing from its rudi- 

 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 compli- 

 cated ; but, within the last half century, the labours of 

 such men as Yon 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 em- 

 bryologist as are the steps of the metamorphosis of the 

 silk-worm moth to the school-boy. It will be useful to 

 consider with attention the 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. 



The Dog's egg is, in fact, a little spheroidal bag (Fig. 

 13), formed of a delicate transparent membrane called the 

 vitelline membrane, and about T -£ ¥ to y^h of an inch in 

 diameter. It contains a mass of viscid nutritive matter — 

 the ' yelk ' — within which is inclosed a second much more 

 delicate spheroidal bag, called the ' germinal vesicle'* (a). 

 In this, lastly, lies a more solid rounded body, termed the 

 1 germinal spot ' (b). 



