252 KANSAS UNIVERSITY SCIENCE BULLETIN. 



the manubrium, which press together the entoderm in the 

 corresponding regions of the umbrella. In the intermediate 

 regions between the ridges of the manubrium the entodermal 

 cell layers are widely separated, forming the four radial 

 canals. 



As the medusa bud grows, the manubrium increases in size 

 until it finally breaks through the outer capsule at the tip, 

 where there are but two layers of cells (figure 9). Further 

 development of the bud involves no radical change in the 

 arrangement of cell layers. The tips of the radial canals 

 widen out and meet to form the circular canal. The apex of 

 the manubrium never opens to the exterior. The sexes are 

 separate, and the germ cells, either eggs or sperm, are de- 

 veloped in the ectoderm of the manubrium, and shed, when 

 ripe, while the medusa is still attached. 



In the fully developed medusas there is a distinct difference 

 between the sexes. The male medusa has a relatively longer 

 manubrium and shorter umbrella than the female medusa, and 

 the manubrium of the male is more blunt at the tip than that 

 of the female. 



Sexual differentiation is not apparent until fairly late stages 

 of development, but the sex of even the very earliest buds 

 may be determined by their relation to the older medusas in 

 which the sex is evident. Since medusae of only one sex are 

 found on a single peduncle, the sex of young buds will always 

 correspond to that of the older medusae borne upon the same 

 peduncle. 



II. — THE ORIGIN, MIGRATION AND DEVELOPMENT OF THE GERM 



CELLS. 



1. — Distinguishing characters of early germ cells. The 

 earliest germ cells to be found in Corymorpha palma are 

 marked by certain characters which serve to distinguish them 

 from other cells, entodermal or ectodermal. The nucleus of 

 a primitive germ cell, wherever it may be found, is usually 

 larger than the nuclei of other cells, but this difference in 

 size does not always serve as a means of identification. The 

 nucleolus is large and prominent. The nucleus is much less 

 dense than the surrounding cytoplasm, and the deeply stain- 

 ing chromatin is condensed into coarse masses which lie 

 around the periphery, close to the nuclear wall. The combina- 

 tion of these two characters, the clearness of the nucleus and 



