112 



LEPIDOSTEUS. 



to it. The next stages have not been observed, but on the third 

 day after impregnation (fig. 58), the animal pole is completely 

 divided into small segments, which form a disc similar to the 

 blastoderm of meroblastic ova ; while the vegetative pole, which 

 subsequently forms a large yolk-sack, is divided by a few 

 vertical furrows, four of which nearly meet at the pole opposite 

 the blastoderm. The majority of the vertical furrows extend 

 only a short way from the edge of the small spheres, and are 

 partially intercepted by imperfect equatorial furrows. 



The stages immediately following the segmentation are still 

 unknown, and in the next stage satisfactorily observed, on the 

 fifth day after impregnation, the body of the embryo is distinctly 

 differentiated. The lower pole of the ovum is then formed of a 



mass in which no traces of 



j> 



segments or segmentation fur- ; 



rows can be detected. 



The embryo (fig. 59) has 

 a dumbbell-shaped outline, 

 and is composed of (i) an 

 outer area, with some resem- 

 blance to the area pellucida 

 of an avian embryo, forming 

 the lateral part of the body ; 

 and (2) a central portion con- 

 sisting of the vertebral plates 

 and medullary plate. The 

 medullary plate is dilated in 

 front to form the brain (br). 

 Two lateral swellings in the 

 brain are the commencing 

 optic vesicles. The caudal 

 extremity of the embryo is somewhat swollen. 



Sections of this stage (fig. 60) are interesting as shewing 

 a remarkable resemblance between Lepidosteus and Teleostei. 



The three layers are fully established. The epiblast (ep} is 

 formed of a thicker inner nervous stratum, and an outer flat- 

 tened epidermic stratum. Along the axial line there is a solid 

 keel-like thickening of the nervous layer of the epidermis, which 

 projects towards the hypoblast. This thickening (MC) is the 



FIG. 59. SURFACE VIEW OF A LEPI- 

 DOSTEUS EMBRYO ON THE FIFTH DAI- 

 AFTER IMPREGNATION. 



br. dilated extremity of medullary plate 

 which forms the nidiment of the brain. 



