44 EMBRYOLOGY 



result of a lateral budding, which during its earliest stages 

 pursues a course quite similar to that described above. Here 

 too there is formed at first a small spheroidal bilaminar 

 bud (Fig. 16 A), between the two cell-layers (ectoderm and 

 entoderm) of which there can be recognized a hyaline sus- 

 tentative membrane. The next change, which takes place 

 at the same time with the progressive abstriction of the 

 bud, is the formation of an ectodermal thickening at the 

 free distal pole, which is developed (Fig. 16 B) into a knob, 

 the so-called nucleus of the bud (nucleus of the hell). By the 

 growth of the latter into the interior of the bud, the ento- 

 dermal sac is invaginated, so that it now assumes the form 

 of a cup (Fig. 16 B). While a fissure (the beginning of the 

 cavity of the bell) makes its appearance (Fig. 16 D) in the 

 nucleus of the bud, the two facing layers of the cup-shaped 

 entoderm sac come into close contact, and fuse along 

 four meridians (Fig. 16 E), so that of the cavity of the 

 entoderm sac only four perradial regions (i.e. places cor- 

 responding with the four chief radii) remain open ; they are 

 the fundaments of the four radial canals. It is to be 

 observed that these four radial canals are connected with one 

 another through the remnant of the obliterated entoderm sac 

 (Fig. 16 ^, ^), in other words through the originally bila- 

 minar so-called vascular lamella (cathammal plate) (L. Agassiz, 

 No. 2 ; Glaus, No. 62). While with the further enlargement 

 of the bud the bell-cavity increases in size and breaks through 

 to the outside, and while the manubrium grows out at the 

 bottom of it, the sustentative lamella of the wall of the 

 bell is converted into the gelatinous substance of the 

 umbrella. The radial vessels have become relatively nar- 

 rower and farther separated from one another. The ring- 

 canal at the margin of the bell appears to arise by a 

 secondary separation of the two layers of the vascular 

 lamella. With these metamorphoses and with the breaking 

 through of the mouth-opening and the formation of the 

 velum,^ the medusa is essentially completed and ready to be 

 detached (Fig. 16 F). 



* The velum does not arise by the formation of a fold, but directly 

 from that ectodermal membrane which in early stages (Fig. 16 D) sepa- 



