[WRIGHT] AN EARLY ANADIDYMUS OF THE CHICK 23 



In the trigeminal region of the hi.nd-hrain the neural canal is open for 

 some thirteen sections, but before the auditory region is reached it is 

 again closed as far as section 84, near which point (Fig. 11) there 

 IS again a failure to close for a few sections; thereafter, however, the 

 canal is closed as far as section 120, Fig. 13, behind which point the 

 groove is, at first narrowly, and then widely, open. 



In section 160 (Fig. IG) the fusion of the ventral wall of the 

 neural groove and the notochord begins and is continued in the follow- 

 ing sections (Figs. 17-20), the complete fusion of the ectoderm, chorda, 

 mesoderm and entoderm being attained at the 17oth section (Fig. 20). 

 Beyond this point we can hardly speak of a neural groove; the 181st 

 section (Fig. 21), indeed, shows an unsymmetrical fissure which is not 

 uncommon in the primitive groove of normal embryos, and by section 

 190 all traces of the primitive streak have disappeared and the germinal 

 area presents a normal appearance (Fig. 23). The comparison of my 

 Figures 15-22 with those of Hertwig (1 c. Figs. 536-54:5, page 891) 

 shows that there is little difference except in the less amount of closure 

 of the neural canal, and without an inspection of sections further 

 forward, it would be impossible to detect any symptom of " duplicita*." 



JSTOTOCTTORD. 



The conduct of the two notochords has already been sufifiiciently 

 referred to in the hinder region ; it only remains to call attention 

 to their gradual increase in size from their first appearance in section 

 9 (immediately behind figure 5) till their fusion in section 131, also 

 to their gradual convergence to this point. 



ME,soDErî:\r. 



As already remarked there are ten somites, and this is the case 

 with the "median" series of fused somites which lie exactly in the 

 same plane as the lateral ones. 01' the " median "' series, the seven 

 posterior are better demarcated than those further forward, and ar,j 

 sometimes notched on their ventral surface. The rudiments of the 

 Wolffian body may be seen in the region represented in Figs. 12 and 13. 



Vascular SysTEir. 



A convenient starting-point for the description of the vascular 

 system is the region depicted in Fig. 10 (section 67), Avhere the vitel- 

 line veins are perfectly normal, and the only thing that arrests atten- 

 tion is the ■'■ median " descending aorta. Fig. 9 shows that the vitel- 

 line veins have not become fused into a sinHe heart as in a normal 



