94 



LEPIDOSTEUS. 



only. The nervous layer soon however grows in again from the two 

 sides. 



Where the neural cord is separated from the epidermis, it is 

 already provided with a well-developed lumen. Posteriorly it re- 

 mains in its earlier condition. 



In the region of the hind-brain traces of the auditory vesicles 



are present in the form of 

 slightly involuted thicken- 

 ings of the nervous layer 

 of the epidermis. 



The mesoblast of the 

 trunk is divided anteriorly 

 into splanchnic and somatic 

 layers. 



In the next stage, on 

 the sixth day after impreg- 

 nation (fig. 61), there is a 

 great advance in develop- 

 ment. The embryo is con- 

 siderably longer, and a great 

 number of mesoblastic so- 

 mites are visible. The 

 body is now laterally com- 

 pressed and raised from the 

 yolk. 



The region of the head 

 is more distinct, and late- 

 rally two streaks are visible (br.c), which, by comparison with the 

 Sturgeon, would seem to be the two first visceral clefts 1 : they are 



not yet perforated. In the 

 lateral regions of the trunk 

 the two segmental ducts are 

 visible in surface views (fig. 

 01, sd) occupying the same 

 situation as in the Sturgeon. 

 Their position in section is 

 shewn in fig. 62, sg. 



With reference to the fea- 

 tures in development, visible in 

 sections, a few points may be 

 alluded to. 



The optic vesicles are very 

 prominent outgrowths of the 

 brain, but are still solid, though 

 the anterior cerebral vesicle has 

 a well-developed lumen. The 



FIG. 01. EMBRYO OF LEPIDOSTEUS ON THE 

 SIXTH BAY AFTER IMPREGNATION. 



op. optic vesicles ; br.c. branchial clefts (?); sd. 

 segrnental duct. 



N.B. The branchial clefts and segrneutal duct 

 are somewhat too prominent. 



1X1 3 



FIG. 62. SECTION THROUGH THE TRUNK OF 

 A LEPIDOSTEUS EMBRYO ON THE SIXTH PAY 

 AFTER IMPREGNATION. 



me. medullary cord; ms. mesoblast; 5/7. 

 segmental duct; ch. notcchord; ;r. sub-noto- 

 chordal rod ; liy. hypoblast. 



I have as yet been unable to make out these structures in section. 



