210 



ROBERT PAYNE BIGELOW ON 



— d 



g- 



-g 



noted that the larvae produced by budding are set free in a form resembling planulae, 



and it is the planula stage in sexual reproduction which is the earliest that can be 



compared with any stage to be described in this paper. 



All agree that the planula of the Scyphomedusae is a more or less oval, ciliated 



larva, somewhat flattened on one side. It consists of two layers of cells surrounding a 



cavity which is completely shut off from 

 the exterior. The first step toward the 

 development of the scyphistoma is an in- 

 vagination of the ectoderm at the posterior 

 pole of tlie planula. According to Goette 

 and Miss Hyde, the endoderm is invagina- 

 ted at tlie same time in such a way that 

 two endodermal pouches remain in the 

 plane of the long diameter, — " Haupte- 

 bene," Goette, — (Fig. B, and h-b Fig. 

 D), one on each side of the invagination, 

 while the endoderm is pushed entirely 

 away from the oral pole in the plane of 

 the short diameter (Fig. C, and c-c Fig. 

 D). The ectodermal invagination is the 

 oesophagus (Schlund) , its external open- 

 iui!; is the mouth, and the endodermal 

 evaginations are the first pair of gastric 

 pouches. Soon an opening is formed 

 (Schlundpforte) between the base of the 

 oesophagus and the central stomach, and 

 at the edge of the opening the ectoderm 

 fuses with the endoderm. Then the sec- 

 ond pair of gastric pouches is formed. 

 According to Goette these are produced in 

 Cotylorhiza and Pelagia entirely' from the 

 ectoderm of the lower part of the oesopha- 

 gus (Fig. F) . According to his view the 

 lower edges of these evaginations coincide 

 with the margin of the gastro-oesophageal 

 opening (Schlundpforte) , and are therefore 



at the level of the upper edges of the first pair of pouches. As these pouches extend out- 



Figs. B-G. Two stages in tlic development of tlie mouth 

 and gastric pouches in a sexually produced scyphistoma, 

 according to Goette. Figs. B to D are sections in the three 

 dimensions of space of a larva in which the oesophagus is 

 invaginated and the first pair of gastric pouches are formed. 

 Figs. E to G are similar sections of a larva in which the open- 

 ing from the oesophagus into the stomach (Schlundpforte) has 

 been established and the second pair of gastric pouches are in 

 the process of formation. 6, c, li, etc., indicate the plane of 

 Figs. B, C, D, etc. 



