DEVELOPMENT OF THE EMBRYO. 



1001 



first trace of the Glottis appears about the fifth day ; it is then a mere slit in 

 the walls of the ossophagus, resembling that by which the ductus pneumaticus 

 of some Fishes opens into the alimentary canal. The formation of the cartila- 

 ginous rings of the trachea does not commence until after the twelfth day, when 

 they first appear as transverse strise on the median line of the front only ; they 

 gradually become solid, and extend themselves on either side, until at last they 

 nearly meet on the median line, on the back or vertebral side of the tube. 

 The history of the process, in the Human embryo, appears to be very nearly 

 the same. The first appearance of the Lungs takes place at about the 6th 

 week, at which time they are simple elevations of the external layer of the 

 03sophageal wall. Their surface, however, soon becomes studded with numerous 

 little wart-like projections ; and these are caused by the formation of correspond- 

 ing enlargements of their cavity. These enlargements soon become prolonged, 

 and develop corresponding budlike enlargements from their sides; and in this 

 manner, the form of the organs is gradually changed, a progressive increase in 

 their bulk taking place from above downwards, in consequence of the extension 

 of the bronchial ramifications from the single tube at the apex. At the same 

 time, however, a corresponding increase in the amount of the parenchymatous 

 tissue of the lung is taking place ; for this is deposited in all the interstices 

 between the bronchial ramifications, and might be compared with the soil filling 

 up the spaces amongst the roots of a tree. It is in this parenchyma that the 

 pulmonary vessels are distributed ; and the portion of it whrch extends beyond 

 the terminations of the bronchial tubes seems to act as the nidus for their 

 further extension. It can be easily shown that, up to a late period of the 

 development of the lungs, the dilated terminations of the bronchi constitute the 

 only air-cells (Fig. 271, c) ; but, as already mentioned, the parenchyma sub- 

 sequently has additional cavities formed within it. It is a fact of some interest, 

 as an example of the tendency of certain diseased conditions to produce a return 

 to forms which are natural to the fostal organism, or which present themselves 

 in other animals, that up to a late period in the development of the Human 

 embryo, the lungs do not nearly fill the cavity of the chest, and the pleura of 

 each side contains a good deal of serous fluid. 



1007. The embryological development of the Urinary organs in Vertebrated 

 animals is a subject of peculiar interest ; owing to the correspondence which 

 may be traced between the transitory forms they present in the higher classes, 

 and their permanent condition in the lower. In this 

 respect there is an evident analogy with the Respi- 

 ratory system. The first appearance of anything 

 resembling a Urinary apparatus in the Chick, is seen 

 on the second half of the third day. The form at 

 that time presented by it, is that of a long canal, 

 extending on each side of the spinal column, from 

 the region of the heart, towards the allantois (Fig. 

 269, o, o); on the sides of this are a series of ele- 

 vations and depressions, indicative of the incipient 

 development of cseca. On the fourth day, the Cor- 

 pora Wolffiana, as they are then termed, are dis- 

 tinctly recognized as composed of a series of csecal 

 appendages which are attached along the whole course 

 of the first-mentioned canal, opening into its outer 

 side (Fig. 272, 1). On the 5th day, these append- 

 ages are convoluted; and the body which they form state of the Urinary and Genital 

 acquires increased breadth and thickness. They Apparatus in the early embryo of 



,, , . , , , J the Bird : 1, corpora Wolffiana : 



evidently then possess a secreting function, and the 2> 2> their excretory ducts . 3j kid . 

 fluid which they separate is .poured by their long neys; 4, ureter; 5,5, testis. 



