256 



ZOOLOGY 



SECT. 



irregularly circular or elliptical space filled in by a mass of large 

 cells the yolk-plug (C, yk.pl?). Soon, however, this wide aperture 

 becomes narrowed to a small longitudinal slit, the lips of the 

 anterior part of which then unite, only the most posterior part 

 remaining open (D) and subsequently giving rise to the anus. 

 During its increase in size the blastopore has been growing over 

 towards the dorsal side, and when its lips become united, it 

 extends along the greater part of the dorsal surface. A narrow 

 medullary groove (E, Up. sut.) appears along the dorsal surface, 

 and a pair of medullary folds are seen at the sides of this (E) 



yk.pl 



FIG. 912. Ceratodus forsteri. Stages in the development. A, lens-shaped blast ula ; B, 

 stage with semicircular blastopore (I>1. p.) ; C, later stages in which the blastopore (bl. p.) has 

 taken the form of a ring-like groove enclosing the yolk-plug (ylk. pi.) ; Z>, stage in which the 

 narrow medullary groove (hip. sut.) has appeared with the rudiment of the medullary folds 

 (med.) ; E, stage in which the medullary folds (med.) have become well developed ; h\ later 

 stage with well-formed head with two visceral arches (rise.) and rudiments of eye (< j yf) and 

 ; pron. mesonephros. (After Semon.) 



and are coalescent in front of it. From the medullary folds 

 and the groove between them the neuroccele, and subsequently 

 the entire nervous system, are developed as in Craniata in general 

 (see p. 08). The portion of the blastoderm destined to give rise 

 to the embryo becomes to a slight extent folded off from the 

 rest, which forms an ill-defined rounded mass, or yolk-sac, to be 

 subsequently absorbed as development proceeds. The most 

 important features in the later stages (F) are the negative ones 

 of the absence of the external gills (to be referred to below 

 and in the account of the Amphibia), and the absence of horny 

 jaws. 



