417 



plasm may still maintain a union (Fig. 6). In the more anterior part 

 of the trunk, where separation is completed, one or both surfaces have 

 here and there smoothed down their outlines (Fig. 7). Compare the 

 shape of the hypochord in this figure with its shape in Fig. 6. This 

 spreading-out and flattening has been caused before separation by 

 the thinning of the hypoblast. It will be noted that "Haemkeimzellen" 

 destined to form the dorsal aorta are moving into place. 



The notochord is at a less distance from the gut in the bran- 

 chial region than in the trunk, and thus we pass from an area in 

 which the hypochord has completed its separation to one in which 

 this is only progressing, and from this to one in which the hypochordal 

 rudiment forms a ridge upon the hypoblast (Fig. 8). Vacuoles and, 



^^^— -> ao h 



Zeilen. 



Fig. 7. Fig. 8. 



Fig. 6. 5.5 mm embryo. Through posterior umbilical region, y^ 400. 



Fig. 7. 5.5 mm embryo. Through middle trunk region. ^ 400. ao haemkeim- 

 1. 



Fig. 8. 5.5 mm embryo. Through branchial region. X 400. 



I think, also intercellular spaces are abundant in some parts of the 

 ridge and in the hypoblast below, there being other signs of degener- 

 ation in the latter. Do any of these vacuoles and spaces, lying 

 alone in the centre of the hypochord as they sometimes do, constitute 

 the lumina of Stöhr? 



In the auditory region is a short gap where hypochordal cells 

 are absent. The ridge begins again in front of the gap; but soon 

 the hypochord projects forward, freed from the hypoblast, as a short 

 process which terminates opposite to the anterior borders of the audi- 

 tory placodes and just behind the anterior ends of notochord and gut. 

 This freedom of the anterior end of the hypochord is due to the round- 

 ing off of the anterior end of the gut and is not a result of ad- 

 vanced development. The separate part in front of the gap is the 

 "head part" of Stöhr, but I am unable to attach to it any morpho- 

 logical significance as a separate structure. Why, will later be seen. 



Another 5.5 mm embryo has no gap in the hypochord, which extends 

 unbroken throughout the body as far forward as in the last embryo. 



Anat. Adz. ZXXV. Aufsätze. 



27 



