122 EMBRYOLOGY 



genital hand (Fig. 62 G), which is now developing as a fokl 

 of the gastral wall. The sexual products arise from elements 

 of the wall of this fold, are ripened (Fig. 62) between its 

 two lamellae, and by the dehiscence of the wall pass into the 

 gastral cavity, whence they reach the outside world through 

 the mouth. The space underlying this fold and communi- 

 cating with the gastral cavity is called the genital sinus 

 (Fig. 62 Gs). The genital band, which is usually horse- 

 shoe-shaped, is often interrupted at the interradius, so that 

 we then find- in the four interradii eight paired gonads, 

 which are often more or less adradially placed, a condition 

 which probably must be looked upon as the primitive one. 

 With the progressive increase in the thickness of the 

 mesogloea, which grows, principally at the four corners of 

 the mouth, into massive pillars, there is in the interradial 

 region a more and more marked development of an invagina- 

 tion of the outer surface of the body, which is called the 

 sub-genital cavity (Fig. 62 S), and in its earliest beginnings 

 is perhaps to be referred to the cavity of the septal infundi- 

 bula. While, accordingly, the body-wall of the medusa is 

 thickened by an increase of the mesogloea all around this 

 place, it here remains as a very thin g astro- genital membrane 

 (Fig. 62 Gm), which in many forms (e.g. Pelagia) shows a 

 tendency to protrude outwai'ds like a hernial sac, so that 

 in this way a genital sac {g astro- genital pouch) projecting into 

 the sub-genital sinus is developed. 



While one might conclude from the structure of the adult genital band 

 that it was developed by a simple folding of the sub-umbrellar wall of 

 the stomach, the investigations of v. Lendenfeld and Hamann show 

 that the earliest fundament of the genital band is merely a thickening 

 of this wall, and that an elevation of this thickening in the form of a 

 fold does not take place until later, when an invagination pushes forward 

 more and more from the distal side, thus producing the genital sinus. 



General Considerations. — The fact that in the eggs 

 of most Scyphomedusae a Scyphistoma stage is first de 

 veloped, and that this stage is also indicated in the modified 

 development of Pelagia (Goette), shows that we must 

 imagine the ancestral form of the Scyphozoa as an attached 

 Anthozoa-like polyp, which originally possessed, in addition 



