LEPIDOSTEUS. 



already provided with a well-developed lumen. Posteriorly it 

 remains in its earlier condition. 



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

 are present in the form of slightly involuted thickenings of the 

 nervous layer of the 

 epidermis. 



The mesoblast of 

 the trunk is divided 

 anteriorly into splanch- 

 nic and somatic layers. 



In the next stage, 

 on the sixth day after 

 impregnation (fig. 61), 

 there is a great advance 

 in development. The 

 embryo is considerably 

 longer, and a great num- 

 ber of mesoblastic so- 



FIG. 61. EMBRYO OF LEPIDOSTEUS ON THE 

 SIXTH DAY AFTER IMPREGNATION. 



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

 segmental duct. 



N.B. The branchial clefts and segmental duct 

 are somewhat too prominent. 



mites are visible. The 

 body is now laterally 

 compressed and raised 

 from the yolk. 



The region of the head is more distinct, and laterally two 

 streaks are visible (br.c\ mc 



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. 61, sd] occupying the 

 same situation as in the Stur- 

 geon. Their position in sec- 

 tion is shewn in fig. 62, sg. 



With reference to the features 

 in development, visible in sections, 

 a few points may be alluded to. 



FIG. 62. SECTION THROUGH THE TRUNK 

 OF A LEPIDOSTEUS EMBRYO ON THE SIXTH 

 DAY AFTER IMPREGNATION. 



me. medullary cord ; ms. mesoblast ; sg. 

 segmental duct ; ch. notochord ; x. sub-noto- 

 chordal rod ; hy. hypoblast. 



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



