424 THE REPRODUCTIVE FUNCTIONS. 



but, in birds, they arc soon folded together into a rounded 

 mass; while, in the mean time, the two filaments of the 

 spinal cord have approached each other, and united into a 

 sino-le column, the form which they ever after retain. Even 

 at this early period the rudiments of the organs of the higher 

 senses, (first of the eye, and next of the labyrinth of the ear,) 

 make their appearance: but, on the other hand, those of the 

 leo-s and wings do not show themselves until the brain has 

 acquired greater solidity and development. The nerves 

 which are to connect these organs of sensation and of mo- 

 tion with the spinal cord and brain are formed afterwards, 

 and are successively united to the nervous centres. 



Although the plan of the future edifice has thus been 

 sketched, and its foundations laid in the homogenous jelly 

 by the simpler efforts of the vital powers, the elevation of 

 the vast superstructure demands the aid of other machinery, 

 fitted to collect and distribute the requisite materials. Here, 

 then, we might, perhaps, expect to meet with a repetition 

 of those vegetative processes, having similar objects in view, 

 and the adoption of analogous means for their accomplish- 

 ment; but so widely different in character is the whole or- 

 ganic economy of these two orders of beings, that we per- 

 ceive no resemblance in the mechanism employed for their 

 formation. For the purposes of animal life the nutrient 

 juices must be brought into active circulation by means of 

 vessels extensively pervading the system. Nature, then, 

 hastens to prepare this important hydraulic apparatus, with- 

 out which the work of construction could not proceed. 

 What may be the movements of the transparent nutrient 

 juices at the very earliest period must, of course, remain 

 unknown to us, since we can only follow them by the eye 

 after the nutritive substance they contain has become con- 

 solidated in the form of opaque globules. These globules 

 are at first seen to meander through the mass, unconfined by 

 investing vessels; presently, however, a circular vessel is 

 discovered, formed by the foldings of the membrane of the 

 embryo, along which the fluids undulate backwards and for- 



