THE DEVELOPMENT OF HYDRA. 273 



remain attached to the egg until after the third cleavage by 

 means of a connecting thread of cytoplasm. When the eggs are 

 not fertilized they disintegrate and finally fall to pieces. 



The cleavage is total, unequal and regular. The cleavage 

 cells communicate with each other by means of protoplasmic 

 bridges or connections. There is a passage of substance from 

 one cleavage cell to the other. The cleavage cavity begins with 

 the third cleavage. The blastula when completely formed is a 

 hollow sphere of primitive ectodermal cells. 



The origin of the endoderm is multipolar. The outer and inner 

 egg membranes are formed from the ectoderm ; the first is formed 

 by an outgrowth of the ectodermal cells, the second by means 

 of a secretion from the same cells. The inner ends of the ecto- 

 dermal cells persist and become the definitive ectoderm. The 

 gastrula consists of a solid spherical mass of cells surrounded by 

 the egg membranes. The eggs are either glued to the object on 

 which the parent rests or remain attached to the parent until the 

 embryo hatches. The eggs of Hydra sp. ? will not continue to 

 develop when removed from the parent after fertilization occurs, 

 or even after cleavage has begun. 



Hydras seldom continue to reproduce by budding after the 

 sexual generation is completed. Exhaustion due to vigorous 

 budding precedes the appearance of the sexual organs, more 

 especially the ovaries. 



Hydra sp. ? reproduces by budding during the entire year. 

 No sexual organs were found on the hydras when collected, but 

 after the animals had been kept in aquaria with abundance of 

 food, sexual organs appeared on those hydras that had been 

 budding vigorously for several weeks. Spermaries or ovaries 

 never appeared on buds. 



It gives me pleasure to express my gratitude to Dr. George 



Lefevre for reading this manuscript. 



Zoological Laboratory, 



University of Missouri, 

 January 30, 1908. 



