250 KANSAS UNIVERSITY SCIENCE BULLETIN. 



peduncle (figure 1) . This stretching and thinning of ectoderm 

 and mesoglea continues as development proceeds (figure 3). 



Further development of the medusa is shown in figure 3. 

 The bud has elongated and there is a decided thickening of the 

 ectoderm at its apex. The cells here are crowded together 

 and the cytoplasm has rounded up around the nucleus so as to 

 give a dense appearance to the whole region. The ectoderm of 

 the sides and base of the bud consists of a single layer of 

 cells. The cells of the entoderm have come to lie in a row 

 just beneath the mesoglea, those at the tip being more defi- 

 nitely organized than the others farther back. A loose mass of 

 structureless cytoplasm occupies the center of the bud, and a 

 cavity is forming just beneath the well-organized cells at the 

 apex. 



The next stage (figure 4) shows the origin of a structure to 

 which Weismann applied the term "glockenkern." Weismann 

 says, "Als Entocodon oder Glockenkern bezeichne ich jenes 

 wichtige Embryonalorgan, durch dessen Vermittlung die 

 einfach blindsackformige Knospe zur Meduse umgewandelt 

 wird, jene schon von so vielen Autoren gesehene und unter 

 verschiedenen Bezeichnungen beschriebene Wucherung des 

 Ektoderms in der Spitze der Knospe, welche den Ento- 

 dermschlauch eindruckt und so zur Bildung einer becher — 

 oder kelch — formigen hohlen Entoderm — Duplikatur Anlass 

 giebt, aus welcher die Entodermlage der Medusenglocke 

 hervorgeht. Es is notig, dafur eine bestimmte, einfache und 

 unzweideutige Bezeichnung zu haben."* This "glockenkern" 

 originates from the thickened mass of ectodermal cells at the 

 tip of the bud. The ectoderm and entoderm have separated 

 in this region and some of the ectoderm cells have passed into 

 the cavity formed by this separation. The "glockenkern" cell 

 mass thus formed later gives rise to the subumbrella and the 

 ectoderm of the manubrium, which functions as a germ gland. 

 As this "glockenkern" cell mass enlarges it pushes back the 

 layer of entoderm cells beneath it. 



Figure 5 shows the further development of the "glocken- 

 kern." The whole bud has increased in size, and the pear- 

 shaped cell mass has grown down into the central cavity 

 of the bud, pushing back the endoderm as it advances. 

 Germ cells are first seen in the "glockenkern" at this stage, 



* Weismann, August. 1883. Die Entstehung der Sexualzellen bei den Hydromedusen. 



