419 



In front of this section the hypochordal cells gradually decrease 

 in size, or rather, diameter, towards the anterior trunk region. Here 

 some cells are degenerating, and they certainly have shrunk since for- 

 mation. The cells in the branchial region are larger again and healthy, 

 except that they are vacuolated at certain places (Fig. 12). 



And now, with the conditions in this embryo as a basis, we may 

 turn our attention to the question of Stöhr's bridges and to that of 

 the "head part". 



Before the branchial region is reached, above the solid part of 

 the oesophagus, we meet with the remnants of a bridge. Part of it 



— not by any means cylindrical in shape — hangs 

 from the hypochord and bulges downward the 

 dorsal wall of the aorta ; another part remains on 

 the dorsal wall of the oesophagus. The latter 

 part, but not the former, is found at the same 

 place in a number of embryos of this size. In 

 some of them a small heap of cells lies upon 

 the hypoblast in the cloacal region. Another 



heap is in the middle of the tail in an older embryo^ About tMrf 

 embryo. These masses in the tail and cloacal branchial cleft. X^oo. 

 region belong to the chorda-hypoblast ; but what 

 the bridge in the oesophageal region is formed of I am unable 

 to say. 



In the branchial region we again meet with the bridges; yet I 

 cannot describe the conditions existing here in any other than the 

 following terms. When the hypochord is drawing away from the gut 

 in this region, a band of cells — attaining different heights in different 

 areas before it disrupts, and scarcely developing over a certain area 



— maintains a temporary union between the two. Of course, the 

 joining of the two vessels destined to form the dorsal aorta is thereby 

 delayed. The band is derived mainly from hypoblast, described before 

 as being vacuolated and degenerating in some areas within this region 

 in a 5.5 mm embryo ; and it seems to be formed by a process resem- 

 bling a folding. How far it is composed of chorda-hypoblast is a 

 matter of doubt. In this embryo it has already disappeared at one 

 place in front of and behind which it is represented only by fragments 

 of degenerated cells. It has broken away from the hypochord at 

 another place and remains attached to the hypoblast (Fig. 13); at 

 yet another it has separated from the hypoblast and remains attached 

 to the hypochord. Over one area it is still unbroken, and over 

 another it is degenerating (Fig. 12). I may remark incidentally that 



27* 



