404 THE DEVELOPMENT OF THE CHICK 
the condition found in many mammals, the cortical cords of the 
birds corresponding to the cortical substance, and the medullary 
cords to the medullary substance of mammals. The medullary 
cords are often called phaochrome or chromaffin tissue on account 
of the specific reaction of the constituent cells to chromic acid, 
and their supposed genetic relation to tissue of similar composition 
and reaction found in the carotid glands and other organs asso- 
ciated with the sympathetic system. 
hig. 231. — Photograph of a cross-section of an embryo of 8 days through the 
ostia tubs abdominalia. 
a. ALS., Neck of abdominal air-sac. O. T. a., Ostium tube abdominale. 
M’s’t. uc., Accessory mesentery. pl. C. r., 1., Right and left pleural cavities. 
Rec. pn. ent. r., Right pneumato-enteric recess. V. ¢e. a. 1., Left anterior 
vena cava. R., rib. Other abbreviations as before. 
The embryonic history has been the subject of numerous 
investigations, and has proved a particularly difficult topic, if 
we are to judge from the variety of views propounded. Thus 
for instance it has been maintained at various times: (1) that 
