584 JAMES F. GEMMILL. 



DESCRIPTION OF FIGURES. 

 LETTERING EMPLOYED. 



b.cav. Buccal cavity. 



bl. Blastopore (anus of larva). 



entc. Enterocoele. 



ente'. Region of archenteron which produces the enterocoele. 



entc.l. Left enterocoele. 



enter. Right enterocoele. 



m.o. Mouth opening. 



oes. ffisophagus. 



oes'. Region of archenteron which produces the oesophagus. 



po.cil.bd. Postoral ciliated band. 



pr.cil.bd. Preoral ciliated band, 



rect. Rectum. 



stom. Stomach. 



•stem'. Region of archenteron which gives rise to the stomach. 



PLATE I. 



Figs. 1-4. — Examples in which the archentera from two foci of gastrul'ation have 

 rcmamed separate. In Fig. 1 the blastopores are near one another and the archentera 

 are parallel and equally developed. Compare the bipinnarial stage shown in Fig. 13. 

 In Fig. 2 the blastopores are again near one another, the archentera being unequal ; 

 compare the bipinnarial stage shown in Fig. 14, though in the latter the inequality 

 has manifested itself later and been less pronounced. In Fig. 3 the blastopores are a 

 considerable distance away from one another and the archentera are markedly unequal ; 

 compare also Fig. 7. In Fig. 4 the foci of gastrulation have appeared on ojjposite sides 

 of the larva. 



Figs. 5-8. — Examples in which the blastojjore being single, the archentera are bifid 

 anteriorly. In Fig. 5 the doubUng only affects the anterior or cnterocoelic-cesophageal 

 part of the archenteron ; cf. the bipiimarise shown in Figs. IG and 17. In Fig. 6 the 

 doubling reaches as far back as the commencement of the stomach ; cf. anterior portion 

 of the bipinnaria shown in Fig. 20. In Fig. 7 there is doubling to a like degree and in 

 addition there is a smaU, entirely independent archenteron with its own blastoi:)oric 

 opening on one side ; the larva thus exhibits a modified form of triplicity and is the only 

 triple monster obtained. For parallel instances in Fishes see 6, pp. 33, 35. In Fig. 8 

 the doubling extends as far back as the stomachal region ; cf. posterior half of the 

 bipinnaria shown in Fig. 20. 



Figs. 9-11. — Examples in which there are two separate blastopores, but the archen- 

 tera are united in front to a greater or less degree. In Fig. 9 the archentera are equal 

 and fused only at their extreme anterior ends ; cf. the bipinnaria shown in Fig. 19. 

 In Fig. 10 there is the same condition, but the archentera are unequal. In Fig. 1 1 the union 

 reaches back to the stomachal region. 



