1889.] On the Early Development of Lepidosteus osseus. 



Ill 



Tlie blastopore closes on the second day, and at no time is a canalis 

 neurentericus formed. 



The embryo becomes more and more distinctly marked out, and 

 the nervous system is differentiated. 



By-and-bye the embryo gets raised above the level of the blastoderm, 

 and a solid tail-bud is very early rounded off above what was the 

 anterior margin of the blastopore. 



The mesoderm or mesoblast arises very early, and before the close of 

 the blastopore. 



If embryologists have not yet agreed as to its mode of origin in the 

 chick, how can one hope to settle this question in the difficult material 

 of Lepidosteus ? It appears to me to arise from the epiblast on each 

 side of the middle line, and from the epiblastic region at the lip of 

 the blastopore. 



I make these statements with the utmost diffidence. Apart from 

 its medium thickening, the epiblast is very early divided into two 

 layers. The outer or covering layer (Deckschicht of the German 

 authors) takes no share in organ formation at all. It covers the 

 embryo everywhere, but no organs are formed from it. It may, 

 perhaps, be compared to the skin of a larval Annelid. The inner 

 layer may be spoken of as the formative epiblast. 



The hypoblast has only a dorsal or neural extension, the ventral 

 side of the embryo being occupied by yolk. 



Epiblastic Organs. Nervous System. — This is formed solely out of 

 the formative epiblast, though it must be noted that the outer layer 

 is often grooved in the middle line. It may be regarded as a folded 

 plate of epiblast, which sinks below the rest of the formative epiblast, 

 and it differs solely in its mode of formation from the nervous systems 

 of Blasmobranchii and Amphioxus in that the folds are closely 

 applied to each other, and only separate later on in development. 

 The brain vesicles can soon be distinguished, and in later stages 

 a similar apparent segmentation can be traced for some distance 

 along the spinal cord. The nervous system becomes hollowed oat by 

 the separation of its walls from each other. The optic vesicles arise 

 as a pair of hollow evaginations of the fore-brain. The lens also is 

 a product of the formative epiblast. 



In the central nervous system two structures are very early dis- 

 tinguishable : — 



(1.) The transient giant ganglion cells in the spinal cord ; these are 

 described in a separate section (p. 117). 



(2.) The ciliated groove which I have elsewhere described as form- 

 ing the floor of the primitive central canal. It persists in the adult, 

 and there also forms only the floor of the central canal as in sharks 

 and elsewhere. 



The Brain. — The fore-brain roof in embryo and adult is very thin, 



