CHAP. XLI.] OF THE HUMAN EMBRYO. 593 



fissures were present. Wagner and Muller have examined ova a 

 little further advanced. The embryo examined by the latter ob- 

 server was two and a half lines in length, and the amnion was seen 

 closely applied to it. There were three pair of branchial fissures 

 and arches. The age of this embryo may be stated with tolerable 

 certainty to be about twenty-five days. 



Several human ova have been subjected to careful examination 

 by different observers, between three and four weeks old, which 

 may be considered to be in a normal state, and not modified by the 

 occurrence of any morbid change. At this time, the ovum is about 

 seven lines in length, and the embryo not more than two. It is 

 surrounded by an amnion which adheres to it pretty closely. The 

 chorion is already covered with small villi. Between the chorion 

 and amnion is a considerable quantity of a viscid albuminous 

 material. The embryo is curved. The anterior cerebral vesicles 

 are well marked, and immediately behind them are the very large 

 corpora quadrigemina. The positions of the ophthalmic and auditory 

 vessels are indicated. Three or four visceral arches are seen with 

 the branchial fissures between them. The heart is not yet fully 

 formed, and projects from the anterior surface of the body as a 

 bent tube. The anterior and posterior extremities exist in the form 

 of curved flattish appendages. The abdomen is completely open, 

 and the amiiion is reflected over the anterior and posterior extremity 

 of the embryo to form the cephalic and caudal involucra. 



The heart is large, and consists of a simple auricle and ventricle. 

 Behind the heart is seen the liver, beneath which is situated the 

 intestine attached by its mesentery. The umbilical vesicle is as 

 large as the embryo, and it is connected with the intestine by a 

 long pedicle, the ductus omphalo-entericus. The vesicle itself is 

 found lying between the chorion and amnion (Figs. 280, 294). The 

 duct, with the vessels which accompany it, and the allantois, 

 constitute the umbilical cord, enveloped by the amnion, which 

 membrane forms a tube enclosing these structures. 



The allantois is seen as a large and well-defined vesicle, which 

 extends to the inner surface of the chorion, to which it is attached. 

 On each side of the mesentery, close to the spine, are situated the 

 large Wolffian bodies. 



About the fourth week, there are three branchial fissures and 

 arches behind the lower jaw. The divisions of the vertebrae are 

 very distinctly marked, and in consequence of the curvature 

 of the embryo, the coccyx and forehead are brought very close 

 together. 



