i7oo HUMAN ANATOMY. 



dependent terminal portion does not keep pace with that nearest the intestine. The 

 apical segment of the caecum remains proportionately small, and persists as the ver- 

 miform appendix. The latter, therefore, corresponds to the unexpanded morpho- 

 logical termination of the caecum. This relation is evident at birth, when the appendix 

 forms the direct continuation of the funnel-shaped caecum ; it is exceptionally re- 

 tained in the adult as the foetal type of caecum occasionally observed. Usually the 

 caecum continues to expand with the colon, the demarcation of the appendix be- 

 coming progressively more emphasized, until the relative size of the two tubes com- 

 monly seen is established. The usual displacement of the appendix, so that it arises 

 from the left and posterior wall of the caecum, results from the later unequal expansion 

 of the right side of the latter, whereby the origin of the appendix is pushed to the 

 left. 



Differentiation of the walls of the intestinal tube begins early in the third month 

 by the formation of longitudinal folds, at first in the upper part, later the entire 

 length of the small intestine. These folds increase in number and size, and subse- 

 quently break up transversely into areas from which the villi are formed. The latter 

 first appear in the upper part of the small intestine in embryos of about 30 mm. 

 in length (Berry 1 ), and gradually extend to the lower segments, the villi being 

 present throughout the small intestine in embryos of about 10 cm. in length. Villi 

 also exist temporarily in the large intestine, but later undergo absorption, so that 

 shortly after birth they have completely disappeared, while those within the small 

 intestine have greatly increased in numbers and size. Early in the fourth month the 

 intestinal glands appear in the upper part of the tube as minute diverticula clothed 

 with extensions of the entoblastic lining of the gut. The glands of Brunner develop 

 somewhat later during the same month as outgrowths of the entoblast. During the 

 fourth month the mesoblastic stratum, from which arise all parts of the intestinal wall 

 except the epithelial elements of the mucosa and the glands, undergoes differentia- 

 tion into the muscular and areolar layers ; by the close of the fifth month all coats 

 of the intestine are well defined. 



Differentiation of the Body-Cavity. Owing to the precocious develop- 

 ment of the mammalian heart, the latter organ is formed by the approximation and 

 fusion of two lateral anlages, at first widely separated, in consequence of which union 

 the upper part of the ventral body-wall is closed, while the more caudally situated is 

 still incomplete, the gut-tube being but imperfectly separated from the yolk-sac. 

 With the more advanced closure of the ventral body-wall the abdominal cavity is de- 

 fined. The primary coelom, according to His, may be divided, therefore, into an 

 upper and a lower portion, the parietal and the trunk-cavity respectively. These spaces 

 communicate on either side by an extension of the parietal cavity, the parietal recess 

 of His. The ventral portion of the parietal cavity, which from its earliest appear- 

 ance contains the heart, becomes the pericardial cavity, and is, therefore, appropri- 

 ately named the pericardia! ca'lom (Mall 2 ). The upper part of the parietal recess, 

 since it later contains the lung and forms the greater portion of the surrounding 

 lung-sac, may similarly be designated the pleural ccelom. For a time the separation 

 between the pericardial and pleural coeloms is imperfect, owing to the incompleteness 

 of the postero-lateral walls of the heart-sac. This deficiency is corrected by tin- 

 growth and differentiation of the pulmonary ridge (Mall), a structure that extends 

 from the liver along the dorsal wall of the duct of Cuvier to the dorsal attachment of 

 the early fold suspending tin- In-art, or mesocardium. Mall has shown that the pul- 

 monary ridge gn>\vs headward as tin- plcnro-pericardial f>ic/l>nuit\ which completes 

 the separation between the In -art- and lung-sacs, and later tailward to form the plenro- 

 pt-ritont-al nifi!<ra>t<\ which subsequently aids in closing the communication bet \\een 

 the pleural and peritoneal cavities. 



At first, immediately below the young heart lies the wall of the wide yolk-stalk, 

 embedded within the mesoblastic tissue of which the two large vitelline veins pass in 

 their course towards the lower end of the heart. With the formation of the body- 

 wall and the narrowing of the yolk-stalk, the enlarged vitelline veins, in their journey 

 towards tin- heart, produce a broad fold which projects horizontally into the body- 



1 Anatom. An/ei^er, Hd. xvii., 1900. 



1 Johns Hopkins Hospital Hull, tin, vol. xii., 1901 ; Journal of Morphology, vol. xii., 1897. 



