THE AXIAL GRADIENTS IN HYDROZOA. 

 VI. EMBRYONIC DEVELOPMENT OF HYDROIDS. 



C. M. CHILD, 

 i From the Hull Zoological Laboratory, the University of Chicago.; 



During the summers of 1917 to 1920, spent at the Puget Sound 

 Biological Station, Friday Harbor, Washington, some time was 

 spent in the study of polarity in eggs and embryos of several 

 species "f hydmzoa and of modification and control of develop- 

 ment thron-h differential susceptibility. Continuation, of the 

 work has not been possible since 1920, but the data already at 

 hand an. c< insistent in themselves and constitute further evidence 

 for tin nee and importance in development of the physiologi- 



cal gradients as well as a basis for interpretation in physiological 

 terms of the transformation of the planula into the hydroid form. 

 This opportunity is taken to acknowledge again my obligations *o 

 the Director of the Puget Sound Biological Station for the facili- 

 ties- placed at my disposal. 



MATERIAL AND METHODS. 



!'. . anse cif its abundance the hydromedusa, Pliialidium gre- 

 nstituted the chief source of embryonic material. Since 

 the medusa- shed eggs and sperm more or less continuously in 

 sin. ill quantities, fertilized eggs were most readily obtained in 

 quant us by keeping medusa? in considerable numbers, often sev- 

 eral hundred, in large pans or tubs for a few hours and then 

 collect in- e-gs and embryos from the bottom. In such material 

 stages may range from newly shed eggs to various cleavage stages 

 or early Mastulaj, according to the length of time the medusae have 

 been in the container. Embryonic stages of Stoinotoca atra were 

 obtained in the same way, but the medusae of this species were 

 usually much less abundant than Phialidhnn. Ovarian eggs of 

 all stages nf growth were obtained directly from the ovaries of 

 these species. In a third species, Gonothyraa clarkii, which was 

 u-i-.l to some extent, the free swimming medusa sta.ue i- absent, 



19 



