HUMAN EMBRYO IN THE NINTH STAGE. 



143 



pleure of the embryo to join the omphalo-mesaraic vein and enter the heart. In 

 the figure only the general course of the vein is indicated. The fact that it is 

 situated in the somatopleure could not. well be shown. 



Human Embryo in the Ninth Stage with Three Gill-clefts Showing Externally. 



Our knowledge of this stage is quite good. The described embryos vary in 

 length from 2.6 to 4.2 mm. The chorionic vesicles are about 10 mm. in diam- 

 eter, varying according to the size of the embryo. Figures 88 and 89 

 represent two embryos of this stage, the latter being the more advanced. The 

 back of the embryo is normally (or at least usually) convex. The head is bent 



FIG. 88. HUMAN EMBRYO OF 2.6 MM. LENGTH. (After W. His.) 



to one side, usually to the right, and the tail to the other, the whole embryo hav- 

 ing a slight spiral twist. The embryo has become quite large in proportion to 

 the yolk-sac. The three gill-clefts are readily seen, the first being the largest, the 

 third the smallest. The column of tissue between the first cleft and the mouth is 

 the mandibular process. Between it and the fore-brain lies the shorter rounded 

 maxillary process. The segments are clearly marked externally along the back 

 (Fig. 89). The origin of the amnion is shown in figure 89 also. The heart has 

 grown and something of its more complicated form is indicated in the external 

 modeling of the embryo. The anlage of the future ear is now a closed vesicle or 

 otocyst (Fig. 90, of). From the region over the heart almost to the caudal 

 extremity the segments of the body are distinctly marked externally. 



The general anatomy of this stage will be understood by the aid of the accom- 

 panying figures 90 to 93, which are all reconstructions from sections. The position 

 of the notochord, Ch, is indicated by a line (Fig. 91). The pharynx is large and 



