DEVELOPMENT OF THE LUNGS. 945 



lobe increases a little or remains stationary, whilst the left lobe undergoes 

 an absolute diminution, being reduced nearly one-half; and as, during the 

 same period, the bulk of the rest of the body has been rapidly increasing, 

 the proportion is much more, reduced during that period, than in any sub- 

 sequent one of the same leugth. According to Meckel, the liver of the 

 newly-born infant weighs one-fourth heavier than that of a child eight or 

 ten months old ; and as the weight of the whole body is more than doubled 

 during the same time, it is obvious that the change in the proportion of the 

 two must be principally effected at this epoch. The liver seems to be 

 engaged during foetal life, in the depuration of the blood (as appears from 

 the acumulation of meconium, which is chiefly altered bile, in the intestinal 

 canal at birth") ; but at the same time it is serving as a blood-making organ, 

 containing many free nuclei and protoplasmic masses, 1 and this is probably 

 its principal function before birth. The general history which has just been 

 given of the development of the Liver, seems equally applicable to the 

 other glands that are evolved from the mesoblast and hypoblast forming the 

 parietes of the Alimentary canal, such as the Salivary glands and Pancreas; 1 

 since they all seem to commence in little masses of cells, formed by an in- 

 creased development, at certain spots, of the layers of blastema which origin- 

 ally constituted its wall ; and whilst the cells of the mesoblast give origin to 

 the proper vesicles of each gland, its ducts and tubuli, those of the hypo- 

 blast form their epithelial lining. The development of the Spleen and of 

 the Suprarenal, Thymus, and Thyroid bodies, has been already described 

 (215,217, 220). " 



786. The Lungs are also developed from the mesoblast in immediate re- 

 lation with the upper part of the Alimentary canal, their first rudiments 

 shooting forth as a pair of budlike processes (Fig. 350, a) from the under or 

 anterior surface of the oesophageal portion. These were originally and cor- 

 rectly described by Von Bar as hollow, 



and as being in reality direrticula from FlG - 3 " )0 - 



the tube itself. They gradually in- 

 crease in size, extending downwards 

 by the multiplication of their compo- 

 nent cells in that direction ; and their 

 cavities at first communicate with the 

 pharynx by separate apertures ; these, 

 however, as the channels are elongated 

 into tubes coalesce to form the trachea First a PP earaDCe of the Lun s s: a > in a Fowl at 



^ & ' C , ( ' a > four days ; b, in a Fowl at six days ; c, termina- 



and the pulmonary organs become re- tion of bronchus in a very young i'i g . 

 moved to a distance from their point 



of exit (Fig. 350, b). The first appearance of the Lungs in the Chick takes 

 place towards the close of the third day, and in the Human embryo at about 

 the sixth week. Their surface soon becomes studded with numerous little 

 wartlike projections, which are caused by the formation of corresponding 

 enlargements of their cavity; these enlargements scon become prolonged, 

 and develop corresponding budlike enlargements from their sides ; and in 

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

 crease in their bulk taking place from above downwards, in consequence of 

 the extension of the bronchial ramifications of 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 com- 



1 Schonk, Centralblatt, 1869, p. 865. 



2 See ISchenk, Anal. Phys. Untersuch., Wicn, 1872. Abstract in Centralblatt, 1873, 

 p. 35. 



