92 



LEPIDOSTEUS. 



diameter. They are invested by a tough double membrane, composed 

 of (1) an outer layer of somewhat pyriform bodies, radiately arranged, 

 which appear to be the remains of the follicular cells ; and (2) of an 

 inner zona radiata, the outer part of which is radiately striated, 

 while the inner part is homogeneous. 



The segmentation, as in the Sturgeon, is complete, but approaches 

 closely the meroblastic type. It commences with a vertical furrow at 

 the animal pole, extending through {tbout one-fifth of the circum- 

 ference. Before this furrow has proceeded further a second furrow is 

 formed at right angles 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 par- 

 tially intercepted by imperfect 

 equatorial furrows. 



The stages immediately fol- 

 lowing 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 

 segments or segmentation furrows 

 can be detected. 



The embryo (fig. 59) has a dumbbell-shaped outline, and is com- 

 posed of (1) an outer area, with some resemblance to the area pellu- 

 cida of an avian embryo, forming the lateral part of the body ; and 

 (2) a central portion consisting of the vertebral plates and medullary 

 plate. The medullary plate is dilated in front to form the brain {hr). 

 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, GO) are interesting as shewing a re- 

 markable 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 flattened epidermic 

 stratum. Along the axial line there is a solid keel-like thickening of 

 the nervous layer of the epidermis, which projects towards the hypo- 

 blast. This thickening {MG) is the medullary cord ; and there is no 

 evidence of the epidermic layer being at this or any subsequent period 



Fig, 58, Surface view of the ovum 

 OF Lepidosteus with the membranes re- 

 moved ON THE third DAY AFTER IMPREG- 

 NATION. 



