No. 2.] THE LIFE-HISTORY OF GONIONEMUS. 495 



This discovery made it very convenient for obtaining differ- 

 ent stages of segmentation during day-time. 



The eggs are spherical granular bodies .08 of a mm. in diame- 

 ter, and of light brown color. The nucleus is large and 

 usually lies near the periphery of the freshly deposited ^g^. 

 No egg-membrane could be demonstrated in such eggs until 

 some time after fertilization. As polar bodies were seen on 

 only a few occasions, it is probable that they had formed very 

 early and passed off rapidly. They are minute, and cling 

 together. 



The spermatozoa are conical bodies 3 microns long, and i Yz 

 microns broad, slightly constricted near the posterior end, to 

 which a very delicate tail 70 to 100 microns long is attached. 

 In the constricted portion of the body four deeply stainable 

 bodies may be seen from the end to which the tail is attached. 



Segmentation begins about i J^ hours after deposition of the 

 eggs. It is total and equal — especially in the earlier stages. 

 The first four segmentation planes divide the ^%g into 2, 4, 8, 

 and 16 cells, at intervals of about 45 minutes. From this time 

 on, segmentation is not so regular. The cells become colum- 

 nar, and are arranged around a small cavity giving rise to a 

 blastula. This becomes ciliated and rotates in the egg-mem- 

 brane, in some cases with watch hands, and in others, in an 

 opposite direction. The blastula pushes out one pole some- 

 what, and, working its way out of the egg-membrane, it be- 

 comes a pear-shaped, free-swimming planula, in from 12 to 20 

 hours from the beginning of segmentation.^ The larger end 

 goes foremost. It may persist in this stage for several days 

 or for only 40 hours. Then it elongates, loses its cilia, and 

 attaches by the end corresponding to the larger end of the 

 planula. There now has been a differentiation into ectoderm 

 and endoderm, the latter being formed by multipolar budding 

 of cells in the segmentation cavity. Nettling organs are formed 



their eggs at definite hours of the day. He cites four medusae that deposit eggs 

 regularly at 8 p.m. It would be interesting to learn whether any besides Gonio- 

 nemus can be induced by artificial darkness to deposit during the day-time. 



1 Under certain conditions the ciliated free-swimming stage was entirely 

 omitted, but as this abbreviated development is not yet understood, I defer an 

 account of it until later. 



