[41] EMBRYOGRAPHY OF OSSEOUS FISHES. 495 



onic shield and is continued backwards towards the tail-end of the em- 

 bryo. This splitting- also involves the rim of the blastoderm, which is 

 found to be composed of three strata of cells, viz, the epithelial, the 

 sensory or truly epiblastic, and the lowermost or mesoblastic and hypo- 

 blastic. This relation holds throughout the whole disk except at the 

 tail and rim, where the sensory layer and lower one api)ear to pass over 

 into each other, they appear in fact to be folded upon each other, as 

 shown diagrammatically in Fig. 18 at B and r. The two principal lay- 

 ers are at first quite thick ; as the disk spreads, however, the portions 

 in the vicinity of the embryonic disk alone maintain their original thick- 

 ness to a marked extent, and then only along the axis of the embryo as 

 development advances. The two principal strata in the rim r also re- 

 main thicker, and they are really continuous with those involved in the 

 foruuition of the embryo. As development of the blastoderm advances, 

 however, its rim becomes narrower and a less marked feature as well as 

 somewhat thinner. The segmentation cavity is roofed over by the epi- 

 blast alone, consisting of epithelial and sensory layers only. At first 

 the sensory layer which covers the segmentation cavity is two or more 

 cells deep (Alosa), but later this thins out, so that finally when the 

 blastoderm has entirely inclosed the yelk, it is sometimes quite difdcult 

 to demonstrate positively that there is more than a single layer of cells 

 present. The layers covering the segmentation cavity in the Salmon- 

 oids are thicker than in other species with small ova without a vitelline 

 circulation, and in such forms a mesoblastic stratum may be added to 

 the epiblastic covering of the yelk at a late stage of development. The 

 stratum covering the segmentation cavity in the young codfish just 

 hatched is like that last described. The hypoblast is ditterentiated later 

 and is confounded or blended with the mesoblast u[) to the time when 

 the muscular and the ijeritoneal layers are differentiated, which does 

 not take place till about the time the muscle segments commence to be 

 developed ; even after that time it is somewhat difficult to make out 

 the limits of the hypoblastic layer in sections. 



The foundations of the embryonic structures of the young fish have 

 been laid down with the development of the epiblastic and mesoblastic 

 tracts of tissue, and the events which follow, especially the development 

 of the brain and spinal cord, which together we will call the neurula 

 hereafter, play a most important part in still further modifying the his- 

 tory of the primary layers. It is somewhat diflticult to give a clear ac- 

 count of the development of the neurula. without the aid of figures, but 

 this we will now attempt to do as briefly as possible. 



10.— Development of the cerebko-spinal axis or neurula. 



The development of the brain and nervous system or neurula of the 

 teleostean embryo presents some very remarkable peculiarities, the 

 prin(;ipal of which is that it is at first quite solid and only develops a 

 neural canal at a relatively late stage, or after the neural cord {axen- 



