410 A Three Weeks' Human Embryo 



Models and Drawings. 



A model of the entire specimen was made by tlie Born method/ modi- 

 fied as described by Bardeen.^ 



As the sections were not equally perfect, the best of each group of 

 three was drawn at a magnification of 66%. The sections are lOfx thick, 

 and as but one-third of them was drawn, each wax plate of which the 

 model was made, represents the thickness of three sections, or 30/a. The 

 reconstruction was laid out on an outline enlarged from the photograph 

 made by Dr. Mall.* The completed model has a length of nearly 300 mm, 

 A comparison of the model with the photograph taken before embedding 

 shows that in the process, there had been a shrinkage of 10 to 12%. 



The model was used as a basis for the drawings, the contour line of 

 every fifteenth or thirtieth section being indicated. In this way the 

 different levels can be correlated throughout the set of drawings which 

 represent the model sectioned at different planes. The drawings in each 

 case, were corrected by repeated comparison with the specimen. 



The form of the coelom was obtained by building up the parts cut from 

 the entire model. 



Models at a much greater magnification were made of various details, 

 as the fore-brain, the mesonephros and mesonephric tubules. 



A part of the details are illustrated by reproductions of photographs 

 of sections of the embryo. 



External Form. 



This embryo was photographed by Dr. Mall at three times enlarge- 

 ment. The photograph, published in an article by him,* shows the speci- 

 men from the right side, lying upon the opened chorion and is described 

 as " An embryo three weeks old." He also mentions that the umbilicus 

 is at the right instead of the left, as is the more usual position. In 

 another paper, he shows an enlarged outline drawing." 



The general form is shown in Figs. 1 and la, the body being about once 

 and a half as long as the head and at an angle of about 65° with it. The 

 face is featureless except for the wide slit-like mouth (Fig, 2), At the 

 corner of the mouth (Figs. 2, 5, 6) is a depression with a thin gill-cleft- 

 like membrane. The four gill-clefts are irregularly spaced giving the 



'Born, G., Morph. Jahrb., II, 1876; Arch. f. mikr. Anat., XXII, 1883. 

 'Bardeen, C. R., Johns Hopkins Hosp. Bull., XII, 1901. 

 * Mall, F. P., Welch Anniversary Volume, 1900; also Johns Hopkins Hosp. 

 Reports, IX, 1900. 



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



