GANOIDEI. 105 



medullary plate into the medullary canal. This, as shewn in fig. 

 51, is effected in the usual vertebrate fashion, by the establish- 

 ment of a medullary groove which is then converted into a closed 

 canal by the folding over of the sides. 



The uncovered patch of yolk in the blastoporic area soon 

 becomes closed over ; and on the formation of the medullary 

 canal the usual neurenteric canal becomes established. 



The further changes which take place are in the main similar 

 to those in other Ichthyopsida, but in some ways the appearance 



FIG. 51. TRANSVERSE SECTION THROUGH THE ANTERIOR FART OF AN ACIPENSER 



EMBRYO. (After Salensky.) 



Rf. medullary groove; Mp. medullary plate; Wg. segmental duct; Ch. notochord; 

 En. hypoblast; Sgp. mesoblastic somite; Sp. parietal part of mesoblastic plate. 



of the embryo is, as may be gathered from fig. 52, rather strange. 

 This is mainly due to the fact that the embryo does not become 

 folded off from the yolk in the manner usual in Vertebrates ; and 

 as will be shewn in the sequel, the relation of the yolk to the 

 embryo is unlike that in any other known Vertebrate. The 

 appearance of the embryo is something like that of an ordinary 

 embryo slit open along the ventral side and then flattened out. 

 Organs which properly belong to the ventral side appear on the 

 lateral parts of the dorsal surface. Owing to the great forward 

 extension of the yolk the heart (fig. 52 B) appears to be placed 

 directly in front of the head. 



Even before the formation of the medullary canal the cephalic 

 portion of the nervous system becomes marked out. This part, 

 after the closure of the medullary groove, becomes divided into 

 two (fig. 50 B), and then three lobes the fore-, the mid-, and the 

 hind-brain (fig. 52, A and B). From the lateral parts of the at 

 first undivided fore-brain the optic vesicles (fig. 52 B, Op} soon 

 sprout out ; and in the hind-brain a dilatation to form the fourth 

 ventricle appears in the usual fashion. 



