420 



Fig. 13. 10 mm embryo. In 

 front of second branchial cleft. X '^^^• 

 ao paired dorsal aorta. 



in some embryos of this size the band may be found unbroken of a 

 much greater height than in this one. 



Comparison of the whole of the 10 mm embryos establishes the 

 fact that the points where the band begins its degeneration are sub- 

 ject to much variation in position, and nowhere is there a suggestion 

 of metamerism. 



The band is homologous with the hypoblastic fold in Teleosts 

 that extends throughout the branchial region and a little beyond, with 



the definitive hypoblast below and 

 the hypochordal rudiment above. 

 Both Franz and Sobotta noted the 

 degeneration of parts of it, though 

 Franz saw a metameric arrangement 

 of the remainder where Sobotta did 

 not. The bridge above the oeso- 

 phagus in the 10 mm skate embryo- 

 seems to have arisen independently 

 of the band, which is shorter than 

 in the Teleosts. The outliers in 

 cloacal region and tail are, of course, 

 also of independent origin. 

 From consideration of the bridges we pass to consideration of 

 the "head part". In this 10 mm embryo the hypochord has the full 

 extension without a gap appearing in it; but more posteriorly than 

 is the gap in the 5.5 mm embryo there is an attenuation, where rup- 

 ture would later take place. Various conditions hold in the other 10 mm 

 embryos. The hypochord is unbroken in one of them. In some is a 

 gap where is the above attenuation; but in others the gap is further 

 forward, where it occurs in the 5.5 mm embryo. Gaps are present in 

 one embryo at both places. We may recall how one 5.5 mm embryo 

 had a broken hypochord and the other had it unbroken. In face of 

 all this, to a "head part" morphological value cannot be given. The 

 head increases rapidly in size, and there is no doubt that the hypo- 

 chord, whose cells cease to proliferate soon after separation, ruptures 

 at certain places because it cannot keep pace with this rapid growth. 

 An embryo of length 17 mm is the next to be noticed, and in it 

 the thyroid has just appeared. 



At the posterior end many of the recently-formed chorda-hypo- 

 blast cells are degenerating; but the healthy hypochordal cells in front 

 can be traced in their passage upwards. Yet we soon meet again 

 with degeneration, for in most of the tail they are vacuolated, and 



