748 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



is formed around it. This sheath is probably the commence- 

 ment of the permanent sheath of later stages, but at this 

 stage it cannot be distinguished in structure from a delicate 

 cuticle which surrounds the greater part of the medullary 

 cord. 



The hypoblast has undergone no changes of importance. 



The layers at the posterior end of the embryo retain the 

 characters of the last stage. 



Sixth day after impregnation. At this stage (Plate 34, fig. 8) 

 the embryo is considerably more advanced than at the last stage. 

 The trunk has decidedly increased in length, and the head forms 

 a relatively smaller portion of the whole. The regions of the 

 brain are more distinct. The optic vesicles (op.} have grown 

 outwards so as to nearly reach the edges of the area which forms 

 the parietal part of the body. The fore-brain projects slightly 

 in front, and the mid-brain is seen as a distinct rounded promi- 

 nence. Behind the latter is placed the hind-brain, which passes 

 insensibly into the spinal cord. On either side of the mid- and 

 hind-brain a small region is slightly marked off from the rest of 

 the parietal part, and on this are seen two more or less trans- 

 versely directed streaks, which, by comparison with the Sturgeon 1 , 

 we are inclined to regard as the two first visceral clefts (br.c.}. 

 We have, however, failed to make them out in sections, and 

 owing to the insufficiency of our material, we have not even 

 studied them in surface views as completely as we could have 

 wished. 



The body is now laterally compressed, and more decidedly 

 raised from the yolk than in the previous stages. In the lateral 

 regions of the trunk the two segmental or archinephric ducts 

 (sg.) are visible in surface views : the front end of each is placed 

 at the level of the hinder border of the head, and is marked by 

 a flexure inwards towards the middle line. The remainder of 

 each duct is straight, and extends backwards for about half the 

 length of the embryo. The tail has much the same appearance 

 as in the last stage. 



The vertebral regions of the mesoblastic plates are now seg- 

 mented for the greater part of the length of the trunk, and the 



1 Salensky, " Recherches s. le Developpement du Sterlet." Archives de Biol. 

 Vol. II. 1881, pi. xvn. fig. 27. 



