164 



STUDY OF PIG EMBRYOS. 



desirable. They illustrate the general topographical distribution of the organs. 

 The great volume of the central nervous system as compared with the remaining 

 parts is very striking. Of the other organs, the three which are most conspicu- 

 ous by their size are the heart, liver, and Wolffian bodies. Another striking 

 peculiarity of the embryo is the great diameter of the blood-vessels, and espe- 

 cially of the veins, which are of relatively enormous diameter, being proportion- 

 ately much larger than in the adult. In marked contrast to this is the small 

 diameter of the cavity of the trachea and lungs and of the entire intestinal canal. 

 Figure 101 represents in the main a median section of the embryo together 



xso Oe Coe. 



FIG. 102. ANTERIOR WALL OF THE PHARYNX OF A 



HUMAN EMBRYO OF 3.2 MM. 



1-5, Gill arches; the arches are separated from one another 

 by the entodermal and the corresponding ectodermal 

 gill pouches ; the aortic arches are drawn in dotted 

 lines and arise from the median cardiac aorta. M, 

 Mouth. Oe, Oesophagus. Coe, Coslom. X 5 diams. 

 (After IV. His.) 



FIG. 103. AORTIC SYSTEM OF His's EMBRYO, 

 Bl, 4.25 MM. 



I-V, Aortic arches. Uk, Mandible. Si/, Thy- 

 roid gland. K, Main aorta. P, Pulmonary 

 artery. Lg, Lung. Oe, CEsophagus. X 3 

 diams. (After W. His.} 



with the organs of the right side, but with two exceptions, first, the floor of the 

 pharynx is represented as if cut through considerably to the left of the median 

 plane; second, the heart cut to the left of the median plane. The brain and 

 spinal cord are drawn as if opened to show the modeling of the inner surface of 

 the medullary tube. The pharynx is so drawn as to indicate something of the 

 modeling of its floor surface. The opening of the veins into the heart and of the 

 auricle into the ventricle, and the interventricular orifices, are shown. Of the 

 digestive canal only the entoderm is represented, so that the figure displays the 

 entodermal walls of the oesophagus, stomach, and intestine, and shows the two 



