AORTIC- ARCH SYSTEM IN THE HUMAN EMBRYO. 



93 



the brain. It is probable, however, that the injection method would have shown 

 at least parallel tracts of enlarged channels serving as their precursors, since Sabin 

 found this condition in a 19-somite pig. 



The paired vessels were found to be connected for the first time by anastomoses 

 in the late fourth-arch stage. These are but slightly enlarged capillaries and are 

 most advanced somewhat caudal to the ear vesicles. By this time the cord has a 

 well-developed tract of enlarged capillaries on each side, which limit the correspond- 

 ing plexus ventrally and mark off a ventrolateral non-vascular band along the cord. 

 This arrangement resembles the condition found by Evans on the upper surface 

 of the brain in the formation of the superior sagittal sinus. 



_ Isthmus 

 rhombencephal 



Fio. 26, a and 6. Successive stages in the formation of the basilar artery, partly by connecting up of irregularly 

 alternating segments of the right and left longitudinal neural arteries and partly by a coalescence of 

 the two. a, embryo No. 2841, 4 mm. in length; 6, embryo No. 810, 5 mm. in length. 



In a 4-mm. embryo (No. 2841) the formation of the basilar is well under way 

 and the sixth arch is present. In figure 26 a, drawn from a model, two strong 

 anastomoses can be seen about opposite the otic vesicle and some slender ones 

 lying more caudally. Behind the strong anastomoses the right longitudinal 

 artery has enlarged but the left is still uninterrupted. Cranially the left is not only 

 the weaker but has lost its continuity. This enlargement of irregularly alternating 

 segments of the two longitudinal arteries is what De Vriese and Sterzi found in 

 other forms. 



In the model of a somewhat later stage we find the basilar artery as a single 

 vessel through most of its extent, though one large and several smaller islands are 

 present (fig. 26 b). Between them it lies too far midway of the position of the 



