421 



in the trunk and branchial region this is accompanied by karyolysis. 

 The still continuous part of the hypochord ends at the posterior bor- 

 der of the auditory vesicles; further forwards are only fragments, and 

 one of these fragments is still united to the hypoblast by an attenu- 

 ated remnant of the band. 



The last embryo to receive particular mention is one of length 

 21 mm. The liver is now in evidence. I understand that the neur- 

 enteric canal becomes occluded at this stage, when the "growing 

 point" will cease to exist. The sections end just behind the cloaca. 



Everywhere is degeneration. The hypochord is unbroken as far 

 forward as the posterior end of the branchial region, though degener- 

 ating; and more anteriorly are fragments separated from the notochord 

 by perichordal tissue. It is worth noting that the gut is absent in 

 the few sections behind the cloaca, and yet the hypochord is there 

 present. In Rana Stöhr also found the tail-gut to disappear first. 



In older embryos could be demonstrated the total disappearance 

 of the hypochord in head and trunk, and its gradual enclosure by 

 perichordal tissue while degenerating. As sections of their tails were 

 not available, the fate of the hypochord there is not certain; but the 

 degeneration at the anterior end of the tail in the 21 mm embryo, 

 and the degeneration of the chorda-hypoblast cells at the posterior 

 end in the 17 mm embryo, justifies the assumption that it will there 

 also disappear. The extension posteriorly of the hypochord has not 

 been determined. Now it is true that the chorda-hypoblast cells at 

 the posterior end of the 17 mm embryo are degenerate, especially 

 those which will form the hypochord; yet in it the "growing point" 

 is practically ending its existence, and the tail-gut and notochord 

 cannot receive a much greater accession to their length than will the 

 hypochord. 



In a 21 mm and a 22 mm embryo an exceedingly long strand 

 of cells meanders through the mesenchyme in the branchial region, 

 and extends from the hypoblast to the notochord, ending in a club- 

 shaped process, a hypochordal fragment. 



Soon after separation from the hypoblast the hypochord reaches 

 its maximum in development, and up to this period mitotic figures are 

 seen. Degeneration is a rapid process; and curiously enough, the 

 hypochord disappears sooner io the branchial region than in the trunk, 

 where it first developed, and where karyolysis first appeared. It is 

 gone long before the notochord attains its full size. 



The hypochordal cells receive a supply of yolk equal to that of 

 other cells. Much of this yolk has dissolved before degeneration be- 



