414 A Three Weeks' Human Embryo 



again as it leaves the free mesentery and curves around the end of the 

 ccelom, and unites Avith the allantois to form the wide cloaca which is 

 joined by the Wolffian duct (Figs. 5, 17). 



The cloaca is closed by the anal membrane, a thickened plate with only 

 a slight indentation (Fig. 17). 



Allantois. — The allantois extends as a narrow tube along the abdomi- 

 nal stalk and bending over the caudal end of the ealom (Fig. 5), enlarges 

 to form the bladder as it unites with the cloaca (Fig. 17). 



From the standpoint of the development of the diaphragm Mall"* 

 gives the following description of the organs of the thoracic and abdomi- 

 nal cavities: — "Sections of the embryo -1.3 mm. long (No. 148) show 

 the liver sprouts growing in all directions through the septum trans- 

 versum, encircling and ramifying through the omphalo-mesenteric veins, 

 making a condition slightly in advance of that in His's embryo Lr. The 

 sections of this embryo show clearly, that the heart, lungs, liver, and 

 lower peritoneal cavity arise in tissues surrounded by that portion of the 

 ccelom extending into the head in Embryo XII. . . . The lungs arise 

 when the pericardial ccelom goes over into the pleural, i. e., high up in 

 the region of the head. Immediately on the dorsal side of them is the 

 beginning of the lesser peritoneal cavity, and the intestinal tube struck 

 in this section is the stomach. iVll these structures lie on the cephalic 

 side of the first cervical myotome. Projecting into the peritoneal ccelom, 

 encircling and penetrating the omphalo-mesenteric veins, are the projec- 

 tions of the liver. The two lobes reach from the tip of the lungs and 

 the foramen of Winslow to the point where the entodermal cells of the 

 liver arise from the alimentary canal, or in this case, the duodenum. 

 The lobes of the liver lie entirely within the canals of the ccelom on either 

 side of the head. The caudal ends of these ccelom canals have migrated 

 from opposite the second cervical myotome in Embryo XII, to opposite 

 the second thoracic myotome in Embryo 148. It has moved toward the 

 tail, while the cephalic end of the canal, the pericardial ccelom, has been 

 kinked over to correspond with the growth of the heart. . . . AVe have 

 in the embryo the necessary stage to locate the organs which arise in the 

 neighborhood of the septum transversum, as well as to give the fate of 

 the ccelom in their immediate neighborhood." 



The points of the above description illustrated by the figures are: 

 the penetration of the septum transversum by liver substance and blood- 

 vessels (Fig. 10) ; the continuity of pericardial and abdominal ccelom 

 (Figs. 2, 11) ; the position of lung, stomach, liver, and it may also be 



'"Mall, F. P., Johns Hopkins Hosp. Bull., XII, 1901. 



