74 HUMAN EMBRYOLOGY. 



of the trachea and the oesophagus, until its cavity is obliterated. When this 

 occurs the pericardial cavity is entirely shut off from the remainder of the coelom, 

 and it becomes a completely closed space (Fig. 93). 



As the closure of the pericardial cavity is taking place two wing-like folds of 

 mesoderm, connected ventrally with the septum transversmn and laterally with 

 the body walls, appear, caudal to the lungs (Figs. 91, 93). These folds are the 

 rudiments of the lateral parts of the diaphragm, and each passes medially until it 

 fuses with the mesoderm of the side wall of the fore-gut and with the dorsal 

 mesentery. When this fusion is completed the cavity of the portion of the coelom 

 surrounding the lung, the original pleuro-pericardial canal, is separated from the 

 more caudal part of the coelom, which now becomes the peritoneal cavity. 



Only the broad outlines of the processes by which the pleuro-peritoneal canals 

 are separated from the pericardium and the peritoneum are mentioned in the 

 preceding paragraphs. The details of the processes are too complicated for 

 description in an ordinary text-book of anatomy. 



The Formation of the Diaphragm. There are four main parts of the 

 diaphragm, a ventral, a dorsal, and a right and a left lateral. 



The ventral part is formed from the septum transversurn, which is gradually 

 differentiated into a caudal, an intermediate, and a cephalic part. The caudal part 

 is transformed into (1) the mesodermal tissue of the liver, which grows towards 

 the abdomen, (2) the falciform and coronary ligaments, and (3) the small omenturn. 

 The cephalic part becomes the caudal or diaphragmatic wall of the pericardium. 

 The intermediate part is transformed into the ventral portion of the diaphragm. 



The dorsal part of the diaphragm is developed from the mesoderm of the dorsal 

 mesentery of the fore-gut. Each lateral part is derived from a lateral ingrowth 

 which springs ventrally from the septum transversum and laterally from the body 

 wall. The two lateral portions grow towards the median plane till they fuse 

 with the dorsal portion ; but in some cases, especially on the left side, the fusion 

 is not completed. In such cases an aperture of communication remains, between 

 the pleural and the peritoneal cavities, through which a portion of the abdominal 

 contents may pass into the pleural sac, constituting a diaphragmatic hernia. 



SUMMAEY OF THE EXTERNAL FEATUKES OF THE HUMAN EMBEYO 

 AND FOETUS AT DIFFERENT PERIODS OF DEVELOPMENT. 



During the first fourteen days after the impregnation of the ovum the human zygote 

 descends through the uterine tube, assumes the morula condition, enters the uterus, 

 penetrates into the decidua compacta, and differentiates into three vesicles and a mass of 

 primitive mesoderm ; but, probably, it is not until the beginning of the third week, if 

 Bryce's calculations are correct, that a definite embryonic area is present. By that time 

 the zygote is an ovoid vesicle measuring 2'4 by 1-8 mm. Its wall is formed by the 

 trophoblast, and it contains two inner vesicles, the ecto-mesodermal and the entodermal 

 vesicles. The inner vesicles are surrounded by a mass of primary mesoderm in which 

 the extra-embryonic portion of the coelom is beginning to appear. At this period the 

 embryonic area is the region where the walls of the two inner vesicles lie in relation with 

 one another, and it is *19 mm. long (Fig. 30). 



By the eighteenth or nineteenth day the area has attained a length of 1-17 mm. and 

 it is -6 mm. broad. It is pierced, about the centre of its length, by the neurenteric canal ; 

 the primitive streak has appeared on the dorsal surface of the area ; the primitive groove 

 is distinct, and the neural groove is indicated. The body-stalk is bent dorsally, at right, 

 angles with the area, and it contains the allantoic diverticulum, which has already been 

 projected from the wall of the entodermal vesicle (Fig. 95). 



During the next twenty-four hours the length of the embryonic area increases to 1*54 

 mm.; the neurenteric canal is moved caudally, to a point well behind the middle of the 

 length of the area, and the posterior part of the area is bent ventrally, forming the 

 posterior boundary of the hind-gut region and indicating the position of the future cloacal 

 membrane. The head fold has begun to form, and the pericardial region lies in the 

 ventral wall of the rudimentary fore-gut (Fig. 96). 



By the middle of the third week the head and tail folds are distinctly formed andl 



