946 



OF GENERATION EMBRYONIC DEVELOPMENT. 



pared 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 por- 

 tion of it which 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. 350, c) ; but, as already 

 mentioned, the parenchyma subsequently 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 foetal 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. 



787. The embryological development of the Urinary organs in Verte- 

 brated 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 Respiratory system. The first appearance of 

 anything resembling a Urinary apparatus in the Chick, is seen towards the 

 close of the second day in the form of a group of cells projecting on either 

 side from the mass of uncleft mesoblast on the outside of the protovertebrse 

 into the somewhat triangular space formed by the epiblast above, the upper 

 and outer angle of the protovertebrae on the inside, and the mesoblast on the 

 outside. In consequence of the cells assuming a radiated arrangement and 

 separating at their common point of junction, a canal closed anteriorly is 

 formed, which extends from about the fifth protovertebra towards the posterior 

 extremity of the embryo, and quickly descends towards the pleuroperitoueal 

 cavity. On the second half of the third day the duct appears as a long 

 canal, extending on each side of the spinal column, from the region of the 

 heart, towards the allantois (Fig. 348, o, o, seen also in transverse section 



in Fig. 334, uiiy) ; on the sides of this are a 

 series of elevations and depressions, indicative of 

 the incipient development of caeca. On the 

 fourth day, the Corpora Wolffiana, as they are 

 then termed, are distinctly recognized as com- 



L, U5^<5 ^=SV^ posed of a series of csecal appendages, clearly 



Lri ^U I Jp^il'l' 1 developed from the middle lamina of the blasto- 



'M -*?--53l f>_<y f;-^ / Jt , . * 1 / TV r>r>j i i 11 



dermic vesicle ( r ig. oo4), which are attached 

 along the whole course of the first mentioned 

 canal, opening into its outer side (Fig. 351, a). 

 On the fifth day these appendages are convoluted, 

 and the body which they form acquires increased 

 breadth and thickness ; they evidently then pos- 

 sess a secreting function, and the fluid which 

 they separate is poured by their long straight 

 canals (b, b) into the two horns of the cloaca; 

 and between their component shut sacs numbers 

 of small points appear, which consist of little 

 clusters of convoluted vessels, exactly analogous 

 to the Corpora Malpighiana of the true kidney. 

 These bodies remain as the permanent urinary 

 organs of Fishes ; but in the higher Vertebrata 

 they give place to the true Kidneys, the development of which commences 

 in the Chick about the sixth day. The first part of these bodies to be formed 

 is their duct or ureter, which begins to appear about the 80-100th hour, and 



FIG. 351. 



State of the Urinary and Geni- 

 tal Apparatus in the early em- 

 bryo of the Bird : 1, corpora 

 Wolifiuna; 2, 2, their excretory 

 ducts; :i, kidneys; 4, ureter; 5, 

 5, testrs. 



