416 



Fig. 4. 4 mm embryo. 

 Through middle trunk 

 region. X ^00. 



we may call the "anterior growing-point" of the skate embryo has 

 ceased to exist. Thus the formation of chorda-hypoblast has stopped, 

 and at the very anterior end it is well on its way out of the hypo- 

 blast and has lost touch with the lumen of the gut, though still un- 

 divided into its notochordal and hypochordal parts. 



Described next is an embryo of length 

 5.5 mm. The pronephros is now developing. 

 Chorda-hypoblast is still being proliferated 

 at the posterior "growing point", where, ex- 

 ceptionally in these skate embryos, there can 

 be observed the stages in its actual folding 

 with the formation of a groove. In the trunk 

 the notochord has so increased its distance 

 from the gut that the hypochordal cells, 

 clinging to it as usual, have separated from 

 the hypoblast to form the definitive hypochord. The neurenteric canal 

 is completed; and as the typical disposition of the primary layers 

 has accordingly been disturbed, this and the older embryos are not 

 so useful as the last one for the determination of the relation between 

 chorda-hypoblast and mesoblast, whether the former is continuous on 

 either side with the latter, or there are cells between. 



Beginning at the very posterior end and following the sections 

 forward, we find the chorda-hypoblast become more and more folded, 

 and correspondingly the resultant groove — which, along with the arch- 

 enteric space below, is comparable with Kuppfee's vesicle of Teleosts 



— at first broad and shallow, becomes 

 deep and narrow. The walls of the groove 

 meet, and eventually it is obliterated. 

 ch.hy fj-jjg chorda-hypoblast can be readily de- 

 marcated as it moves upwards (Fig. 5), for 

 a longer stretch of it is histologically 

 specialised. After the notochord is cut off 

 and is increasing its distance from the 

 gut, the hypochordal residue first becomes 

 a ridge upon the hypoblast and then 

 is seen to be separating. This is hap- 

 pening in the umbilical region, where 

 in the last embryo it lay within the hypoblast. 



It is not common in the sections to find the hypochord leaving 

 the hypoblast in such a way that the adjacent surfaces of the two 

 have even outlines, and here they are ragged, while strands of proto- 





Fig. 5. 5.5 mm embryo. 

 Behind the posterior end of 

 cut-off notochord. X 450. 



