The Development of the AlHgator 297 



cranial flexure and so shows only one region of the 

 embryo, that of the hindbrain ijih), which is here of 

 essentially the same structure as above described. 

 On each side of the hindbrain is a large auditory 

 vesicle (0) ; that on the left is cut through its 

 center and shows the beginning of differentiation, 

 its lower end being thick-walled and rounded, while 

 its upper end is more pointed and has a thin, some- 

 what wrinkled wall. The notochord {nt) is slightly 

 larger than in the more anterior sections. Numer- 

 ous blood-vessels (hv, ar) are seen in the mesoblast. 

 The pharynx (/?/?) is here open ventrally and also 

 through the gill cleft of the left side; on the right 

 side the plane of the section did not pass through 

 the external opening of the cleft. The mesoblast 

 of the visceral folds is much more dense than 

 that of the dorsal region of the section. 



Figure i^d, as is evident, is a section through 

 the region of the heart, which appears as three ir- 

 regular cavities (///) with fairly thick mesoblastic 

 walls {mes') lined with endothelium {en'). The 

 body wall, though consisting of but little besides 

 the ectoderm (ec), completely surrounds the heart, 

 and the pericardial or body cavity thus formed 

 extends dorsally as a narrow space on either side of 

 the foregut, giving the appearance of a rudimentary 

 mesentery, though no especial development of such 

 a structure would naturally be expected in this 

 region of the embryo. The foregut {ent) is a 

 moderately large cavity lined with a very distinct 



