3IO The Allig-ator and Its Allies 



from its being very closely packed with corpuscles, 

 so that at first glance, under low magnification, it 

 looks more like a nerve than a blood-vessel. 



Figure lyb is also through the pharyngeal re- 

 gion, a short distance behind the preceding section. 

 The growth of the cerebral hemispheres {ch) is 

 better shown than in the preceding figure, as is 

 also the general form of the optic cup {oc). On 

 the left the nasal cavity {n) is seen as an elongated 

 slit with thick walls ; it is cut near, but not through, 

 its opening to the exterior. The same gill cleft 

 {g) that was seen in the preceding figure is seen 

 here as a narrow, transverse cleft, open at both 

 ends. Between the notochord (;//) and the spinal 

 cord {sc) is the same, though now double, blood- 

 filled vessel {hv) that was seen in the preceding 

 section. The other blood-vessels are larger here 

 than in the more anterior region. There is a 

 faint condensation of mesoblast in the neighbor- 

 hood of the notochord, and a more marked conden- 

 sation {mp) farther toward each side is the curiously 

 shaped muscle plate. 



Figure 17c is through the heart region, and that 

 organ is cut through the opening from the lower or 

 ventricular into the upper or aturicular chamber. 

 The thickening of the wall of the ventricle, which 

 was noticed in the preceding stage, has increased to 

 such an extent that there is now a marked differ- 

 ence in the thickness of the ventricular and auric- 

 ular walls. As in the preceding stage, the body 



