746 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



swelling, the boundary between the notochord and hypoblast 

 becomes indistinct A short way behind this point (Plate 35, 

 fig. 21), the notochord unites with the medullary keel, and a 

 neurenteric cord, homologous with the neurenteric canal of other 

 Ichthyopsida, is thus established. In the same region the boun- 

 dary between the lateral plates of mesoblast and the notochord, 

 and further back (Plate 35, fig. 22), that between the mesoblast 

 and the medullary keel, becomes obliterated. 



Fifth day after impregnation. Between the stage last de- 

 scribed and the next stage of which we have specimens, a con- 

 siderable progress has been made. The embryo (Plate 34, figs. 

 6 and 7) has grown markedly in length and embraces more than 

 half the circumference of the ovum. Its general appearance is, 

 however, much the same as in the earlier stage, but in the 

 cephalic region the medullary plate is divided by constrictions 

 into three distinct lobes, constituting the regions of the fore- 

 brain, the mid-brain, and the hind-brain. The fore-brain (Plate 

 34, fig. 6,f.b.} is considerably the largest of the three lobes, and 

 a pair of lateral projections forming the optic vesicles are 

 decidedly more conspicuous than in the previous stage. The 

 mid-brain (m.b,} is the smallest of the three lobes, while the 

 hind-brain (/z..) is decidedly longer, and passes insensibly into 

 the spinal cord behind. 



The medullary keel, though retaining to a great extent the 

 shape it had in the last stage, is no longer completely solid. 

 Throughout the whole region of the brain and in the anterior 

 part of the trunk (Plate 35, figs. 23, 24, 25) a slit-like lumen has 

 become formed. We are inclined to hold that this is due to the 

 appearance of a space between the cells, and not, as supposed by 

 Oellacher for Teleostei, to an actual absorption of cells, though 

 we must admit that our sections are hardly sufficiently well pre- 

 served to be conclusive in settling this point. Various stages in 

 its growth may be observed in different regions of the cerebro- 

 spinal cord. When first formed, it is a very imperfectly defined 

 cavity, and a few cells may be seen passing right across from 

 one side of it to the other. It gradually becomes more definite, 

 and its wall then acquires a regular outline. 



The optic vesicles are now to be seen in section (Plate 35, 

 fig- 2 3 P-} as flattish outgrowths of the wall of the fore-brain, 



