413 



Description of Material. 

 The embryos are described in order corresponding to their sizes; 

 but as the absence of histological differentiation in the youngest em- 

 bryo makes it impossible to determine the early history of the hypochordal 

 cells, it will be well to obtain a glimpse at the knowledge gained 

 from the next older embryo. What occurs in the skate is what Hatta 

 found in Petromyzon. The hypochord is derived from two groups of 

 cells lying originally in the roof of the archenteron at either side of 

 the notochordal rudiment, between it and the mesoblastic rudiments. 

 By the union of these two masses is formed the hypochord. 



This union takes place during that folding of the chorda-hypo- 

 blast ^) whereby its lateral parts are brought together. It is true that 

 a groove does not develope during the folding, but the process is 

 unmistakable. The lateral cells of the chorda-hypoblast move from 

 either side towards the middle line while the median cells are moving 

 upwards, both movements such as occur during the course of a fold- 

 ing. Next, the lateral cells, after meeting in the middle line, where 

 they compose temporarily the median part of the gut roof, begin to 

 move upwards behind the median cells. And hypoblast cells from 

 either side — the mesoblast being already excluded — pass median- 

 wards to take their place in the roof. While the chorda-hypoblast is 

 going upwards out of the hypoblast, two lateral furrows develope in it 

 which presage the cutting-off of a notochordal part from a ventral 

 hypochordal residue; and this residue, as the notochord increases 

 its distance from the gut, clings to the former and therefore neces- 

 sarily separates from the hypoblast. When it separates, it becomes 

 the definitive hypochord. During the folding there is a change of 

 shape in the hypochordal cells, so that their longer axes, lying at 

 first in the plane of a transverse section, become parallel with the 

 longitudinal axis of the embryo; and this latter disposition is the 

 principal guide in identification of hypochordal cells when they are 

 within the hypoblast. 



The youngest embryo described is 2.5 mm in length. The uprais- 

 ing of the embryo out of the blastoderm has begun; but while there 

 is a short head fold and the hypoblast anteriorly is almost completely 

 rounded off into the tubular gut, posteriorly it is still flat. The central 

 nervous system is in the neural plate stage. 



In the anterior trunk region — the part of the embryo to devel- 



1) A term here applied to that part of the archenteric roof that 

 lies between the mesoblastic diverticula; and included in its scope are 

 both notochordal and hypochordal elements. 



