134 DEVELOPMENT. 



development a vertical slit appears between them ; this gradually widens and 

 becomes the opening by which the common cavity of the nose and mouth commu- 

 nicates w r ith the pharynx. The common cavity is afterward divided into nose 

 and mouth by the development of the palate, in the manner spoken of above. 



The tongue appears about the fifth week as a small elevation behind the 

 inferior maxillary arch, to which a pair of elevations, arising from the junction 

 of the third and fourth pharyngeal arches, is united. The line of union of the 

 three elevations is indicated by the V-shaped groove in which the circumvallate 

 papillae are situated. The epithelial layer is furnished by the epiblast. The 

 tonsils appear about the fourth month. 



The anus is also formed by an inflection of the epiblast, which extends inward 

 to a slight extent, and approaches the termination of the hind-gut and finally com- 

 municates with it by a solution of continuity in the septum between the two. The 

 persistence of the foetal septum at either the buccal or anal orifices constitutes a 

 well-known deformity imperforate oesophagus or imperforate rectum, as the case 

 may be. 



The liver appears after the Wolffian bodies, about the third week, in the form of 

 a bifid process, projecting from the intestine at that part which afterward forms the 

 duodenum. This process grows rapidly, its terminal lobes branching abundantly 

 to form a complicated tubular gland. The duct of the gland becomes the main 

 duct of the liver, while the lobes become transformed into the right and left 

 lobes of the liver and surround the vitelline and, later, the umbilical veins, which 

 break up into a capillary plexus and ramify in their substance. About the third 

 month the liver almost fills the abdominal cavity. From this period the relative 

 development of the liver is less active, more especially that of the left lobe, which 

 now becomes smaller than the right ; but the liver remains up to the end of foetal 

 life relatively larger than in the adult. 



The gall-bladder appears about the second month, as an extension of the cavity 

 from which the main duct of the liver is developed ; and bile is detected in the 

 intestines by the third month. 



The pancreas is also an early formation, being far advanced in the second 

 month. It, as well as the salivary glands, which appear about the same period, 

 originates in a projection from the hypoblastic canal, which afterward forms a 

 cavity, and the lobules of the gland are developed from the ramifications of this 

 cavity. The projection for the pancreas appears on the dorsal wall of the intes- 

 tine, while that for the liver is on the ventral surface, and the ducts of the 

 two glands are at first usually separated. During development the duct of the 

 pancreas shifts its position toward the ventral surface and finally, as a rule, joins 

 that from the liver. 



The spleen is entirely of mesoblastic origin, as it originates from the mes- 

 enteric fold which connects the stomach to the vertebral column (mesogastrium). 



Development of the Respiratory Organs. The lungs appear somewhat later 

 than the liver. They are developed from a small median cul-de-sac or diverticu- 

 lum from the upper part of the fore-gut, immediately behind the fourth visceral 

 cleft, as a projection from the epithelial and fibrous laminae of the intestines. 

 During the fourth week a pouch is formed on either side of the central diver- 

 ticulum, and opens freely through it into the fore-gut (pharynx). From these, 

 other (secondary) pouches are given oft', so that by the eighth week the form of the 

 lobes of the lungs may be made out. The two primary pouches have thus a 

 common pedicle of communication with the pharynx. This is developed into the 

 trachea (Fig. 97, B), the cartilaginous rings of which are perceptible about the 

 seventh week. The parts which afterward form the larynx are recognized as 

 early as the sixth week viz. a projection on either side of the pharyngeal open- 

 ing, which is the rudiment of the arytenoid cartilage and a transverse elevation 

 from the third pharyngeal arch, which afterward becomes the epiglottis ; the 

 vocal cords and ventricles of the larynx are seen about the fourth month. Traces 

 of the diaphragm appear in the form of a fine membrane, separating the lungs 



