HUMAN EMBRYO IN THE NINTH STAGE. 



139 



having a slight spiral twist. The connection of the yolk-sac with the embryo 

 has diminished in size, so that it may be said to be connected by a narrower 

 process or neck with the body of the embryo. Although the head and tail ends 

 of the embryo have become further differentiated, it should be noticed particu- 

 larly that there is now a rounded mass which begins between the mouth-cavity 

 and first gill cleft and extends ventral wards between the mouth and the heart, 

 forming a rounded protuberance. This mass of tissue between the mouth and 

 first gill cleft is known as the mandibular process, 

 because it is the anlage of the mandibular region. 

 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. 

 From the region over the heart, almost 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 accompanying fig- 

 ures. Figure 76 is a reconstruction from sections. 

 The position of the notochord, Ch, is indicated by 

 a line. The pharynx is large and wide. It has 

 three lateral outgrowths on each side, 1,2,3, the gill 

 pouches. In front and near the cephalic end of 

 the notochord there is a small median outgrowth, 

 the anlage of the hypophysis. Toward the neck 

 bend the pharynx becomes narrower and passes 

 over into the small entodermal tube, from which 

 we can detect the outgrowth, Lu, which repre- 

 sents the commencing formation of the lungs. 

 This narrow tube leads to the space above the 

 yolk-sac, Yk. s. Just where it passes into the 

 yolk-sac the entoderm has formed the rudiment of 



the liver, Li. Figure 102 gives a view of the anterior wall of the pharynx of 

 another embryo. In front is the large opening of the mouth, M, the oral plate 

 between the mouth-cavity and the entodermal canal having disappeared. 

 This embryo being a little older, the traces of the four gill clefts can already be 

 seen, and there are four entodermal gill pouches. The aortic vessels are indi- 

 cated by dotted lines. The cardiac aorta reaches the pharynx between 

 the bases of the second and third gill arches, and divides into two branches 



FIG. 75. HUMAN EMBRYO 4.2 MM. 



Yks, Yolk-sac. Am, Amnion. All, 



Body-stalk. (After W. His.} 



