DEVELOPMENT OF PETROMIZON FLUVIATILIS. 211 



have a different structure and arise from the ventral unseg- 

 roented part. 



The blood-corpuscles arise from the ventral free edges of the 

 mesoblast, before they unite in the ventral middle line, they 

 collect in a large sinus just behind the heart. The heart 

 appears in the ventral mesentery, formed by the union of the 

 lateral mesoblastic plates ; at first its lumen is continuous with 

 the sinus just mentioned. This sinus lies between the hypo- 

 blastic yolk-cells and the epiblast; it subsequently acquires 

 walls and forms part of the subintestinal vein. 



The ciliated funnels of the pronephros are left as apertures 

 by the segmental duct which in its anterior end is formed from 

 a groove. The groove closes up at intervals, leaving four or five 

 openings which become the funnels. They do not arise as 

 blind projections from the duct, which subsequently, acquire 

 ciliated openings. From the first the pronephros has a 

 double blood supply, pure blood from the aorta passing to the 

 glomerulus, and impure blood in the cardinal veins surrounding 

 the tubuli. 



The early development of the skeleton is described up to the 

 stage where Professor Parker commenced his researches. 



The canal of the central nervous system developes after the 

 neural chord has separated off from the epidermis ; it does not 

 appear to be lined by any invaginated epidermis, as Calberla 

 and Scott maintained. 



The first sign of differentiation of the parts of the brain is 

 the formation on the sixteenth day of the optic vesicles and 

 pineal gland. The division into fore-, mid-, and hind-brain 

 appears soon after, but the fore- and mid -brain are not sepa- 

 rated by any well-marked groove. The first transverse com- 

 missure to appear is situated just in front of the stalk of the 

 pineal gland. It forms the superior commissure of Osborn. 

 Afterwards the ganglion cells thicken round it and form the 

 asymmetrical ganglia habenulse. 



The ganglia on the fifth, seventh, ninth, and tenth nerves 

 are derived from epiblastic thickenings. Their roots probably 

 arise as outgrowths from the neural ridge. The ganglion of the 



